Fix warnings from Rust 1.80 by rely on docsrs --cfg provided by docs.rs

[Mingun]

Serde maintainer say that docs.rs defines --cfg=docsrs when building documentation: serde-rs/serde#2745

Fixes new warning: https://blog.rust-lang.org/2024/05/06/check-cfg.html

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.90%. Comparing base (5d76174) to head (3996459).
Report is 13 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   61.24%   61.90%   +0.66%     
==========================================
  Files          39       39              
  Lines       16277    16520     +243     
==========================================
+ Hits         9969    10227     +258     
+ Misses       6308     6293      -15     

Flag	Coverage Δ
unittests	61.90% <100.00%> (+0.66%)	⬆️

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