Remove direct dependency on libdart in CI

[azeey]

🦟 Bug fix

Fixes #

Summary

Since libdart is a dependency of libgz-physics, we shouldn't have to depend on it directly. Currently, the direct dependence is causing build failures https://github.com/gazebosim/docs/actions/runs/6384535565/job/17327390515 (you'll have to read the logs to see the failure).

#24 7.289 The following information may help to resolve the situation:
#24 7.289 
#24 7.289 The following packages have unmet dependencies:
#24 7.377  libdart6.13-external-convhull-3d-dev : Breaks: libdart-external-convhull-3d-dev (< 6.14.0) but 6.12.1+dfsg4-11build2 is to be installed
#24 7.381 E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

This also removed ci-focal since it's not needed anymore.

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 #2187 (cbb939c) into gz-sim8 (18aa48f) will decrease coverage by 0.12%.
The diff coverage is n/a.

❗ Current head cbb939c differs from pull request most recent head 22b8b99. Consider uploading reports for the commit 22b8b99 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim8    #2187      +/-   ##
===========================================
- Coverage    66.00%   65.88%   -0.12%     
===========================================
  Files          323      323              
  Lines        30719    30719              
===========================================
- Hits         20275    20240      -35     
- Misses       10444    10479      +35     

see 2 files with indirect coverage changes