Remove Bionic from future releases (Garden+)

[chapulina]

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

Summary

sdformat13 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-commenter]

Codecov Report

Merging #878 (f96865e) into main (f174efc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #878   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files          92       92           
  Lines       14223    14223           
=======================================
  Hits        12609    12609           
  Misses       1614     1614           

Impacted Files	Coverage Δ
usd/src/UsdTestUtils.hh	99.00% <ø> (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 f174efc...f96865e. Read the comment docs.