Disable Focal on Harmonic

[azeey]

🎉 New feature

Summary

Harmonic only supports Jammy so removing Focal CI.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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 #363 (48383e4) into main (572f59b) will increase coverage by 0.27%.
The diff coverage is n/a.

❗ Current head 48383e4 differs from pull request most recent head ee12391. Consider uploading reports for the commit ee12391 to get more accurate results

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
+ Coverage   95.43%   95.71%   +0.27%     
==========================================
  Files          10        9       -1     
  Lines        1030     1073      +43     
==========================================
+ Hits          983     1027      +44     
+ Misses         47       46       -1     

see 7 files with indirect coverage changes

[azeey]

@mjcarroll I've disabled doxygen for autogenerated protobuf files in ee12391. The generation used to work somewhat in ign-msgs8 (https://gazebosim.org/api/msgs/8/namespaceignition_1_1msgs.html), but seems to be broken in gz-msgs9 because the generated files are now in the details namespace and our doxygen configuration ignores that namespace. I don't think it's worth the effort to generate docs since the API is documented in https://protobuf.dev/reference/cpp/api-docs/