Remove Bionic from future releases (Garden+)

[chapulina]

• See https://github.com/ignition-tooling/release-tools/issues/597

Summary

There are no plans to include ign-tools2 on Garden, but whenever it's officially released, it won't have Bionic support.

Test it

No Bionic builds should be triggered.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [x] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [x] 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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[scpeters]

this needs https://github.com/ignitionrobotics/ign-tools/pull/79