release-25.1.0-rc: upgrades: lock rows before backfilling them during upgrade

[blathers-crl]

Backport 1/1 commits from #141420 on behalf of @dt.

/cc @cockroachdb/release

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Release note (bug fix): A step in the 25.1 upgrade finalization process that required backfilling jobs now uses locks to ensure it makes progress even when there is contention on the jobs table to prevent the possibility of becoming stuck under heavy load.

Fixes: #140967
Epic: none.

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Release justification: fixes a bug that can in some conditions prevent an upgrade from 25.1 from finalizing, which makes it a potentially severe bug.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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

[cockroach-teamcity]

This change is 

[celiala]

(release-25.1.0-rc branch is frozen - we can temporarily thaw it in the morning)

[celiala]

FYI: I'm going to backfill "Fixes #140967", so that our [newly enabled!] "fix version mapping" system links this backport and issue to v25.1.0.