Pin protobuf to 3.20.*

[AdeelH]

Overview

This fixes the error described in: protocolbuffers/protobuf#10051

CI builds were breaking during the unit testing step with that error.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

scripts/unit_tests should work.

[codecov]

Codecov Report

Merging #1412 (a071f7d) into master (4619494) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1412      +/-   ##
==========================================
- Coverage   71.36%   71.35%   -0.02%     
==========================================
  Files         173      173              
  Lines        8315     8315              
==========================================
- Hits         5934     5933       -1     
- Misses       2381     2382       +1     

Impacted Files	Coverage Δ
...ervision/pytorch_learner/classification_learner.py	73.56% <0.00%> (-1.15%)	⬇️

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