Remove Bionic from future releases (Garden+)

[chapulina]

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

Summary

ign-physics6 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 #331 (522c4bd) into main (224a7e0) will decrease coverage by 0.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #331      +/-   ##
==========================================
- Coverage   76.07%   75.61%   -0.46%     
==========================================
  Files         129      128       -1     
  Lines        5726     5541     -185     
==========================================
- Hits         4356     4190     -166     
+ Misses       1370     1351      -19     

Impacted Files	Coverage Δ
dartsim/src/JointFeatures.cc	58.24% <0.00%> (-6.60%)	⬇️
dartsim/src/ShapeFeatures.cc	36.66% <0.00%> (-3.71%)	⬇️
dartsim/src/SDFFeatures.cc	62.71% <0.00%> (-0.18%)	⬇️
tpe/lib/src/Shape.hh	100.00% <0.00%> (ø)
include/ignition/physics/detail/Joint.hh	100.00% <0.00%> (ø)
include/ignition/physics/detail/Shape.hh	100.00% <0.00%> (ø)
...clude/ignition/physics/detail/ContactProperties.hh
...nclude/ignition/physics/detail/RelativeQuantity.hh	91.95% <0.00%> (+0.09%)	⬆️
dartsim/src/SimulationFeatures.cc	90.16% <0.00%> (+4.76%)	⬆️

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[chapulina]

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

The warnings on Ubuntu Jenkins aren't new.

I think this is good to go!