This change is 

my plan is to fast-follow removing the cluster setting and just version-gate instead, but using the setting initially for ease of backport when/if we decide that is a good idea.

@ajwerner you able to take a look at the job adoption fix in first commit?

could benefit from a test

BACKUP tests, nearly all of them that use schedules, fail without it after the second commit, so it is covered, just didn't see a easy exiting test to extend for targeted coverage, thus including it here rather than its own PR.

@ajwerner actually, that ended up really easy to test, added 415dc1a#diff-5584d9441c078735c97a5b2b11007753c943dbb959afa049a3a5c22e884e59a3

TFTR!

bors r+

Build succeeded:

• GitHub CI (Cockroach)

blathers backport 21.2

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

error creating merge commit from 415dc1a to blathers/backport-release-21.2-76670: POST https://api.github.com/repos/cockroachlabs/cockroach/merges: 403 Resource not accessible by integration []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2 failed. See errors above.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is otan.}