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

[Xiang-Gu]

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.

[cockroach-teamcity]

This change is 

[ajwerner]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @Xiang-Gu)

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pkg/sql/logictest/testdata/logic_test/alter_primary_key line 1645 at r1 (raw file):

# were rewritten, and hence dropping a column from the old PK columns does not
# unexpectedly drop an existing secondary index.
subtest regression_#84040

Can you add a test that explicitly tests the hash sharded index case?

Code quote:

# The following regression test makes sure that when the new PK columns
# is a (strict) subset of the old PK columns, all existing secondary indexes
# were rewritten, and hence dropping a column from the old PK columns does not
# unexpectedly drop an existing secondary index.
subtest regression_#84040

[postamar]

Reminder: https://cockroachlabs.atlassian.net/wiki/spaces/CRDB/pages/73072807/Git+Commit+Messages#Commit-title

[Xiang-Gu]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)

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[postamar]

@Xiang-Gu don't forget to backport this one manually as well.