release-23.1: sql: only validate new regions when adding/dropping

[rafiss]

Backport 1/1 commits from #113956.

/cc @cockroachdb/release

Release justification: low risk fix for a serious bug

---

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.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
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• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[rafiss]

let's wait for #113301 to be backported before merging this.

[chrisseto]

and it looks like the linked PR has been backported!