Remove Bionic from future releases (Garden+)

[chapulina]

• See Garden support gazebo-tooling/release-tools#597

Summary

ign-msgs9 will be on Garden.

Test it

No Bionic builds should be triggered.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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.

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[codecov]

Codecov Report

Merging #229 (bb465b4) into main (8462930) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #229   +/-   ##
=======================================
  Coverage   85.46%   85.46%           
=======================================
  Files           9        9           
  Lines         956      956           
=======================================
  Hits          817      817           
  Misses        139      139           

Impacted Files	Coverage Δ
src/Utility.cc	82.28% <0.00%> (ø)

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[chapulina]

I cancelled the Windows build because the buildfarm is swamped and that shouldn't be affected by this PR. I think this is good to go!

[caguero]

LGTM!