Remove Bionic from future releases (Garden+)

[chapulina]

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

• 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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[scpeters]

this needs #79