Make Date header parsing locale-independent

[rustam-gamidov-here]

Parse RFC1123 Date headers with strptime_l using the classic locale and a literal GMT token, and
trim header whitespace before parsing.

Treat invalid/partial parses as missing timestamps and remove the extra GMT offset correction now that UTC parsing is explicit.

Relates-To: HERESUP-57013

[codecov]

Codecov Report

❌ Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.31%. Comparing base (df55ff5) to head (6fe9041).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...k-authentication/src/AuthenticationClientUtils.cpp	55.00%	5 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1663      +/-   ##
==========================================
- Coverage   80.35%   80.31%   -0.03%     
==========================================
  Files         350      350              
  Lines       14039    14051      +12     
  Branches     1514     1517       +3     
==========================================
+ Hits        11280    11285       +5     
- Misses       2146     2150       +4     
- Partials      613      616       +3     

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