third_party/re2: fix is_pod deprecation warning

[Riolku]

std::is_pod is deprecated in C++20. This was the noisiest of the warnings in our codebase.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bb5c8bd) 89.59% compared to head (55400a1) 89.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2262      +/-   ##
==========================================
+ Coverage   89.59%   89.61%   +0.01%     
==========================================
  Files        1016     1016              
  Lines       35806    35806              
==========================================
+ Hits        32081    32086       +5     
+ Misses       3725     3720       -5     

see 3 files with indirect coverage changes

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