release-21.2: sql: ALTER PRIMARY KEY rewrites secondary index if new PK subsets old PK

[Xiang-Gu]

Backport 1/1 commits from #84303.

/cc @cockroachdb/release

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Previously, during a ALTER PRIMARY KEY, if the new PK columns is a
subset of the old PK columns, we won't rewrite existing unique,
secondary index.

This was inadequate because the user might think this column is not used
anywhere and will want to drop it, which will unexpectedly drop the
dependent unique index.

Fixes: #84040

Release note (bug fix): This PR fixed a bug where, in a
ALTER PRIMARY KEY, if the new PK columns is a
subset of the old PK columns, we will not rewrite existing secondary
index, and hence those secondary indexes continue to have some of
the old PK columns in their suffixColumns.

But the user might, reasonably, think those columns are not used anymore
and proceed to drop them. The bug then caused those dependent secondary
indexes to be dropped, unexpectedly for the user.

Release justification: fixed a bug where unique indexes will be silently
dropped after ALTER PRIMARY KEY followed by DROP COLUMN.

[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