fix: modify log level WARN -> DEBUG in key-auth plugin

[lexburner]

Ⅰ. Describe what this PR did

modify log level WARN -> DEBUG in key-auth plugin

Ⅱ. Does this pull request fix one issue?

fix #2274

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.82%. Comparing base (ef31e09) to head (6d77a85).
Report is 509 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2275      +/-   ##
==========================================
+ Coverage   35.91%   45.82%   +9.91%     
==========================================
  Files          69       79      +10     
  Lines       11576    12912    +1336     
==========================================
+ Hits         4157     5917    +1760     
+ Misses       7104     6649     -455     
- Partials      315      346      +31     

see 77 files with indirect coverage changes

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[CH3CHO]

LGTM