release-23.1: schemachanger,scbuild: fix assertion bug

[postamar]

Backport 1 commit from #106175.

/cc @cockroachdb/release

Release justification: important bug fix

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The declarative schema changer builder contains assertions on the state of elements and their targets and these could sometimes fail because the check which verifies that the corresponding descriptors are not already undergoing a schema change was performed later. This commit fixes this by moving the check earlier so the assertion is never reached in those cases.

This commit removes a skipped test which is superseded by a new test in the schemachanger package.

Fixes: #45510
Fixes: #102927
Fixes: #105754

Release note (bug fix): fixed a bug which manifested itself in error messages containing "failed to drop all of the relevant elements" when executing DDL statements with the declarative schema changer. What this really means is that there's a concurrent schema change that's ongoing. Instead we now behave as expected and wait for it to finish.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• 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.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is