Update the Compatibility changes of release-8.5.4

[Frank945946]

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[gemini-code-assist]

Summary of Changes

Hello @Frank945946, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the release-8.5.4.md documentation to clearly outline compatibility changes for the TiDB 8.5.4 release. The changes provide a structured overview of system variable and configuration parameter adjustments, including new additions, default value modifications, and deprecations, ensuring users are well-informed about potential impacts on their deployments, especially those with specialized configurations.

Highlights

• Upgrade Compatibility Guidance: The documentation now includes explicit guidance for upgrading from v8.5.3 to v8.5.4, emphasizing the need for careful review for clusters with customized performance tuning, TiFlash, or TiKV compaction settings due to default value and behavior adjustments.
• System Variable Adjustments and Additions: Key system variables like tidb_mpp_store_fail_ttl (default changed to 0s for faster TiFlash query dispatch) and tidb_replica_read (now strictly for read-only SQL) have been updated. New variables such as tidb_opt_enable_no_decorrelate_in_select, tidb_opt_enable_semi_join_rewrite, and tidb_stats_update_during_ddl have been introduced to enhance query optimization and DDL processes.
• Configuration Parameter Overhauls: Several TiKV configuration parameters, including max-manifest-file-size, grpc-raft-conn-num, and grpc-concurrency, have seen their default values or calculation methods updated. A significant change involves the deprecation of older region-compact-* parameters in favor of a new gc.auto-compaction configuration group for improved automatic compaction control.
• TiFlash Shutdown Control: A new TiFlash configuration item, flash.graceful_wait_shutdown_timeout, has been added to manage the maximum wait time during graceful shutdowns, ensuring ongoing MPP tasks can complete while preventing new ones.

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[gemini-code-assist]

Code Review

This pull request refactors the 'Compatibility changes' section of the release notes for v8.5.4. The changes introduce a 'Highlights' section and convert the lists of system variable and configuration parameter changes into tables, which significantly improves readability. My feedback focuses on adjusting the heading structure to comply with the documentation style guide for proper hierarchy and casing.

[gemini-code-assist]

The heading levels and casing should be adjusted for consistency and to follow the style guide. Highlights, System Variables, and Configuration Parameters should be level-3 headings under Compatibility changes. Also, headings should use sentence case.

As per the style guide:

• Heading Hierarchy (Line 39): Use proper heading hierarchy. These sections are subsections of 'Compatibility changes' and should be level 3 (###).
• Sentence Case (Line 32): Use sentence case for headings (e.g., ## Configure the cluster). 'System Variables' and 'Configuration Parameters' should be 'System variables' and 'Configuration parameters'.

Suggested change

	## Highlights
	- Change the default value of the [`tidb_mpp_store_fail_ttl`](https://docs.pingcap.com/tidb/v8.5/system-variables/#tidb_mpp_store_fail_ttl) system variable from `60s` to `0s`. This means TiDB no longer needs to wait before sending queries to newly started TiFlash nodes, as delays are no longer necessary to prevent query failures. [#61826](https://github.com/pingcap/tidb/issues/61826) @[gengliqi](https://github.com/gengliqi)
	- Newly created v8.5.3 clusters can be smoothly upgraded to v8.5.4. However, this release includes **default value changes and behavior adjustments** for several system variables and configuration parameters that require attention.
	- Most changes are **safe for general upgrades**, but **clusters with customized performance tuning, TiFlash, or TiKV compaction settings should review this section carefully**.
	- Some legacy configuration items are **deprecated and no longer recommended**, and you should **migrate to the new configuration groups**.
	- Starting from v8.5.4, the [`tidb_replica_read`](https://docs.pingcap.com/tidb/v8.5/system-variables/#tidb_replica_read-new-in-v40) system variable only takes effect on read-only SQL statements. This change improves data read safety and reduces overlaps with other features. [#62856](https://github.com/pingcap/tidb/issues/62856) @[you06](https://github.com/you06)
	## System Variables
	- Add the following system variables:
	| Variable | Type | Default Value | Description | PR | Contributor |
	|--------|------|---------------|-------------|----|-------------|
	| [`tidb_mpp_store_fail_ttl`](https://docs.pingcap.com/tidb/v8.5/system-variables/#tidb_mpp_store_fail_ttl) | Default Value Change | `0` | Default value changed from `60s` → `0s`. TiDB no longer waits before sending queries to newly started TiFlash nodes, as delays are no longer necessary to prevent query failures. | [#61826](https://github.com/pingcap/tidb/issues/61826) | [@gengliqi](https://github.com/gengliqi) |
	| [`tidb_replica_read`](https://docs.pingcap.com/tidb/v8.5/system-variables/#tidb_replica_read-new-in-v40) | Behavior Change | `leader` | Starting from v8.5.4, this variable only takes effect on read-only SQL statements. This improves data read safety and reduces overlaps with other features. | [#62856](https://github.com/pingcap/tidb/issues/62856) | [@you06](https://github.com/you06) |
	| [`tidb_opt_enable_no_decorrelate_in_select`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_opt_enable_no_decorrelate_in_select-new-in-v854) | New | `OFF` | Controls whether to decorrelate subqueries in the `SELECT` list. | [#51116](https://github.com/pingcap/tidb/issues/51116) | [@terry1purcell](https://github.com/terry1purcell) |
	| [`tidb_opt_enable_semi_join_rewrite`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_opt_enable_semi_join_rewrite-new-in-v854) | New | `OFF` | Controls whether to rewrite `EXISTS` subqueries. | [#44850](https://github.com/pingcap/tidb/issues/44850) | [@terry1purcell](https://github.com/terry1purcell) |
	| [`tidb_stats_update_during_ddl`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_stats_update_during_ddl-new-in-v854) | New | `OFF` | When enabled, `ADD INDEX` collects statistics during DDL, allowing immediate optimizer usage. May increase DDL time on large tables. | [#57948](https://github.com/pingcap/tidb/issues/57948) | [@terry1purcell](https://github.com/terry1purcell), [@AilinKid](https://github.com/AilinKid) |
	- [`tidb_opt_enable_no_decorrelate_in_select`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_opt_enable_no_decorrelate_in_select-new-in-v854): controls whether to decorrelate subqueries in the `SELECT` list. The default value is `OFF`. [#51116](https://github.com/pingcap/tidb/issues/51116) @[terry1purcell](https://github.com/terry1purcell)
	- [`tidb_opt_enable_semi_join_rewrite`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_opt_enable_semi_join_rewrite-new-in-v854): controls whether to rewrite `EXISTS` subqueries. The default value is `OFF`. [#44850](https://github.com/pingcap/tidb/issues/44850) @[terry1purcell](https://github.com/terry1purcell)
	- [`tidb_stats_update_during_ddl`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_stats_update_during_ddl-new-in-v854): controls whether to enable the embedded Analyze for DDL. The default value is `OFF`. [#57948](https://github.com/pingcap/tidb/issues/57948) @[terry1purcell](https://github.com/terry1purcell) @[AilinKid](https://github.com/AilinKid)
	## Configuration Parameters
	### Highlights
	- Newly created v8.5.3 clusters can be smoothly upgraded to v8.5.4. However, this release includes **default value changes and behavior adjustments** for several system variables and configuration parameters that require attention.
	- Most changes are **safe for general upgrades**, but **clusters with customized performance tuning, TiFlash, or TiKV compaction settings should review this section carefully**.
	- Some legacy configuration items are **deprecated and no longer recommended**, and you should **migrate to the new configuration groups**.
	### System variables
	| Variable | Type | Default Value | Description | PR | Contributor |
	|--------|------|---------------|-------------|----|-------------|
	| [`tidb_mpp_store_fail_ttl`](https://docs.pingcap.com/tidb/v8.5/system-variables/#tidb_mpp_store_fail_ttl) | Default Value Change | `0` | Default value changed from `60s` → `0s`. TiDB no longer waits before sending queries to newly started TiFlash nodes, as delays are no longer necessary to prevent query failures. | [#61826](https://github.com/pingcap/tidb/issues/61826) | [@gengliqi](https://github.com/gengliqi) |
	| [`tidb_replica_read`](https://docs.pingcap.com/tidb/v8.5/system-variables/#tidb_replica_read-new-in-v40) | Behavior Change | `leader` | Starting from v8.5.4, this variable only takes effect on read-only SQL statements. This improves data read safety and reduces overlaps with other features. | [#62856](https://github.com/pingcap/tidb/issues/62856) | [@you06](https://github.com/you06) |
	| [`tidb_opt_enable_no_decorrelate_in_select`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_opt_enable_no_decorrelate_in_select-new-in-v854) | New | `OFF` | Controls whether to decorrelate subqueries in the `SELECT` list. | [#51116](https://github.com/pingcap/tidb/issues/51116) | [@terry1purcell](https://github.com/terry1purcell) |
	| [`tidb_opt_enable_semi_join_rewrite`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_opt_enable_semi_join_rewrite-new-in-v854) | New | `OFF` | Controls whether to rewrite `EXISTS` subqueries. | [#44850](https://github.com/pingcap/tidb/issues/44850) | [@terry1purcell](https://github.com/terry1purcell) |
	| [`tidb_stats_update_during_ddl`](https://docs.pingcap.com/tidb/v8.5/system-variables#tidb_stats_update_during_ddl-new-in-v854) | New | `OFF` | When enabled, `ADD INDEX` collects statistics during DDL, allowing immediate optimizer usage. May increase DDL time on large tables. | [#57948](https://github.com/pingcap/tidb/issues/57948) | [@terry1purcell](https://github.com/terry1purcell), [@AilinKid](https://github.com/AilinKid) |
	### Configuration parameters

References

1. Use proper heading hierarchy (no jumping from ## to ####). ^(link)
2. Use sentence case for headings (e.g., ## Configure the cluster). ^(link)

[ti-chi-bot]

@Frank945946: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	22fb72a	link	true	/test pull-verify

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