bump protobuf extension to 3.20.0

[brettmc]

this should make the build green for 8.1, finally. We can now consider 8.1 supported,
and so it moves out of experimental - future 8.1 breaks will fail the pipeline.

[codecov]

Codecov Report

Merging #656 (4e3c402) into main (e096e52) will not change coverage.
The diff coverage is n/a.

❗ Current head 4e3c402 differs from pull request most recent head d43674a. Consider uploading reports for the commit d43674a to get more accurate results

@@            Coverage Diff            @@
##               main     #656   +/-   ##
=========================================
  Coverage     84.27%   84.27%           
  Complexity     1146     1146           
=========================================
  Files           127      127           
  Lines          2772     2772           
=========================================
  Hits           2336     2336           
  Misses          436      436           

Flag	Coverage Δ
7.4	84.21% <ø> (ø)
8.0	84.27% <ø> (ø)
8.1	84.27% <ø> (ø)

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