ci.yml: disable 20.04 workflow

[scpeters]

🦟 Bug fix

Disable 20.04 GitHub workflow on main since Harmonic is already not supporting 20.04

Summary

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #112 (01d1d89) into main (94faf1e) will increase coverage by 0.66%.
The diff coverage is n/a.

❗ Current head 01d1d89 differs from pull request most recent head 2ed47b1. Consider uploading reports for the commit 2ed47b1 to get more accurate results

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   79.45%   80.11%   +0.66%     
==========================================
  Files           8        8              
  Lines         331      342      +11     
==========================================
+ Hits          263      274      +11     
  Misses         68       68              

see 5 files with indirect coverage changes