release-23.2: sql: only validate new regions when adding/dropping

[blathers-crl]

Backport 1/1 commits from #113956 on behalf of @rafiss.

/cc @cockroachdb/release

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When we added validation logic to make sure every region corresponded to a known node locality, we were a little too aggressive. The validation made it possible to end up in a state where any ALTER..REGION operation could hang. This could happen in a few situations; for example:

• node is restarted with a different locality flag.
• MR cluster is restored into a non-MR cluster.
• c2c streaming is used with a MR source and non-MR destination.

In all these cases, the problem was that the zone configuration could reference a region that no longer has any nodes with the corresponding locality. The validation was too aggressive, since it would validate those regions which already existed in the zone configuration.

Now, only the newly added region is validated.

fixes #113324
fixes #113871

Release note (bug fix): Fixed a bug that could cause ALTER DATABASE ... ADD/DROP REGION to hang if node localities were changed after regions were added.

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Release justification: low risk fix for a serious bug

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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  issues or test-only changes.
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[cockroach-teamcity]

This change is