docs(admin): add upgrade_to_34 release notes page

[whoalin1]

Summary

Closes #15244

Release notes jumped from upgrade_to_33 to upgrade_to_35, so admins planning a 34 upgrade could not tell whether critical steps were missing. Add upgrade_to_34.rst stating that no extra critical steps are required beyond a normal major upgrade, link the changelog + system requirements, and list it in the release notes toctree.

Test plan

• Docs build includes admin_manual/release_notes/upgrade_to_34.rst
• release_notes/index.rst toctree lists 34 between 35 and 33

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AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.

[susnux]

@whoalin1 thank you for the pull request.

It seems this is AI generated, as per AI policies you need to disclose the usage of any AI:
https://github.com/nextcloud/.github/blob/master/AI_POLICY.md#disclosure

[whoalin1]

@susnux thanks for pointing that out, and sorry for missing it. I've added an AI disclosure to this PR and to my other open Nextcloud PRs. To be upfront: these PRs were largely written with AI coding agents.

The commits don't have Assisted-by: trailers yet. I can add them if you want, that just needs a force push. Happy to adjust anything else.

AI disclosure: this reply was drafted with help from an AI assistant.

[susnux]

@whoalin1 yes all commits assisted with AI need the Assisted-by trailer.
Please instruct your AI to follow our AGENTS.md (or the linked AI_POLICY.md).

[whoalin1]

@susnux done, all commits on my open Nextcloud PRs now carry an Assisted-by: Cursor:grok-4.7 trailer (force-pushed, no content changes). Sorry for the noise.

AI disclosure: this reply was drafted with help from an AI assistant.

[skjnldsv]

Thanks, but closing this one. susnux already answered on #15244: 34 has no critical upgrade changes, same as 29, so there is no page for it on purpose.

The page also states that 34 "continues the PHP 8.3–8.5 support window", which isn't true: stable34 still accepts PHP 8.2 (https://github.com/nextcloud/server/blob/stable34/lib/versioncheck.php#L9). 8.2 was only dropped in 35.