Remove Bionic from future releases (Garden+)

[chapulina]

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

Summary

ign-sensors7 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 #205 (ce8bbdb) into main (1133ee0) will decrease coverage by 0.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
- Coverage   71.26%   70.87%   -0.39%     
==========================================
  Files          30       30              
  Lines        3097     3114      +17     
==========================================
  Hits         2207     2207              
- Misses        890      907      +17     

Impacted Files	Coverage Δ
src/ForceTorqueSensor.cc	80.55% <0.00%> (-4.74%)	⬇️
src/ImuSensor.cc	86.77% <0.00%> (-4.53%)	⬇️
src/RgbdCameraSensor.cc	74.89% <0.00%> (-0.33%)	⬇️
src/DepthCameraSensor.cc	73.96% <0.00%> (-0.31%)	⬇️
src/CameraSensor.cc	76.77% <0.00%> (-0.29%)	⬇️
src/ThermalCameraSensor.cc	60.38% <0.00%> (-0.24%)	⬇️
src/WideAngleCameraSensor.cc	0.00% <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!