[OTLP Exporter] Revert Google.Protobuf version bump

[alanwest]

Reverts #4201 based on discussion and proposal here #4396 (comment)

[codecov]

Codecov Report

Merging #4407 (0720829) into main (9d2c31a) will decrease coverage by 0.09%.
The diff coverage is 96.66%.

❗ Current head 0720829 differs from pull request most recent head 70ba52f. Consider uploading reports for the commit 70ba52f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4407      +/-   ##
==========================================
- Coverage   84.91%   84.83%   -0.09%     
==========================================
  Files         306      300       -6     
  Lines       12154    12092      -62     
==========================================
- Hits        10321    10258      -63     
- Misses       1833     1834       +1     

Impacted Files	Coverage Δ
...tryProtocol/Implementation/MetricItemExtensions.cs	86.86% <93.33%> (+0.99%)	⬆️
...metryProtocol/Implementation/ActivityExtensions.cs	96.23% <100.00%> (+0.30%)	⬆️

... and 17 files with indirect coverage changes