chore: remove doc_auto_cfg in volo, volo-http and volo-grpc

[yukiiiteru]

Motivation

Cargo removed doc_auto_cfg in version 1.92.0; adding this item would cause document generation to fail.

ref:

• https://docs.rs/crate/volo-http/latest/builds/2805732
• Implement RFC 3631: add rustdoc doc_cfg features rust-lang/rust#138907

Solution

• Remove them
• Run rustdoc with --cfg docsrs in test cases
• Release a new version to trigger it again

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.41%. Comparing base (2ecf4d4) to head (50afa31).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #635   +/-   ##
=======================================
  Coverage   43.41%   43.41%           
=======================================
  Files         160      160           
  Lines       19372    19383   +11     
=======================================
+ Hits         8410     8415    +5     
- Misses      10962    10968    +6     

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