ci.yml: disable Focal GitHub workflow

[scpeters]

🦟 Bug fix

Disable CI for unsupported platform

Summary

Gazebo Harmonic doesn't support Ubuntu 20.04, so disable the GitHub workflow.

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 #1336 (f0935bb) into sdf14 (013bc33) will increase coverage by 4.89%.
The diff coverage is n/a.

❗ Current head f0935bb differs from pull request most recent head 208d348. Consider uploading reports for the commit 208d348 to get more accurate results

@@            Coverage Diff             @@
##            sdf14    #1336      +/-   ##
==========================================
+ Coverage   87.51%   92.41%   +4.89%     
==========================================
  Files         134      134              
  Lines       17768    17669      -99     
==========================================
+ Hits        15550    16328     +778     
+ Misses       2218     1341     -877     

see 118 files with indirect coverage changes