Remove Bionic from future releases (Garden+)

[chapulina]

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

Summary

ign-launch6 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.

🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸

[codecov]

Codecov Report

Merging #150 (0bcb3e6) into main (51cab20) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #150   +/-   ##
=======================================
  Coverage   32.92%   32.92%           
=======================================
  Files           4        4           
  Lines         817      817           
=======================================
  Hits          269      269           
  Misses        548      548           

Impacted Files	Coverage Δ
src/vendor/backward.hpp	8.55% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 51cab20...0bcb3e6. Read the comment docs.

[chapulina]

I manually cancelled the Windows job because the buildfarm was swamped and this PR shouldn't affect that. I think this is good to merge!