Remove Bionic from future releases (Garden+)

[chapulina]

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

Summary

There are no plans to include ign-plugin2 on Garden, but whenever it's officially released, it won't have Bionic support.

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 #70 (5b0df75) into main (7c81bcb) will decrease coverage by 0.16%.
The diff coverage is n/a.

❗ Current head 5b0df75 differs from pull request most recent head a013ad5. Consider uploading reports for the commit a013ad5 to get more accurate results

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   99.83%   99.67%   -0.17%     
==========================================
  Files          16       17       +1     
  Lines         615      616       +1     
==========================================
  Hits          614      614              
- Misses          1        2       +1     

Impacted Files	Coverage Δ
core/include/ignition/plugin/detail/Factory.hh	94.44% <0.00%> (-5.56%)	⬇️
core/include/ignition/plugin/Factory.hh	100.00% <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 believe this is good to go!