CI: Use Ubuntu 20.04 in GitHub Actions.

[briansmith]

"The Ubuntu 18.04 Actions runner image is being deprecated and will be removed by 12/1/22" according to the GitHub Changelog blog.

[codecov]

Codecov Report

Merging #1524 (300f58c) into main (00fc3f3) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 300f58c differs from pull request most recent head b5d1748. Consider uploading reports for the commit b5d1748 to get more accurate results

@@            Coverage Diff             @@
##             main    #1524      +/-   ##
==========================================
- Coverage   91.47%   91.46%   -0.02%     
==========================================
  Files         128      130       +2     
  Lines       18801    18819      +18     
  Branches      195      195              
==========================================
+ Hits        17198    17212      +14     
- Misses       1568     1572       +4     
  Partials       35       35              

Impacted Files	Coverage Δ
src/aead/shift.rs	37.03% <0.00%> (-2.97%)	⬇️
src/rand.rs	64.63% <0.00%> (-1.65%)	⬇️
src/polyfill/array_flat_map.rs	95.12% <0.00%> (-0.99%)	⬇️
tests/agreement_tests.rs	90.68% <0.00%> (-0.57%)	⬇️
src/aead/aes.rs	80.76% <0.00%> (-0.49%)	⬇️
src/aead.rs	70.00% <0.00%> (ø)
src/digest.rs	97.02% <0.00%> (ø)
src/aead/quic.rs	65.07% <0.00%> (ø)
src/aead/nonce.rs	100.00% <0.00%> (ø)
src/aead/chacha.rs	99.43% <0.00%> (ø)
... and 13 more

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[briansmith]

One problem here is that Firefox and geckodriver aren't in the Ubuntu 22.04 images. In this PR I've just disabled the testing in Firefox. We could move that testing to Windows (like getrandomdid, IIRC) but for now I've just commented out the Firefox testing so that Chrome is the only browser tested. See the comments mentioning Firefox in actions/runner-images#6399 for more info.