release-22.2: opt: fix invalid st_union optimizations

[mgartner]

Backport 1/1 commits from #112729.

/cc @cockroachdb/release

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The st_union aggregate function was incorrectly thought to ignore
duplicate values. That is, given any number of duplicate input values,
it was assumed to produce the same result. This is not the case in both
Postgres with Postgis and CRDB (see #111556).

This incorrect assumption caused the optimizer to perform invalid
optimizations and produce query plans that return incorrect results.
This commit fixes the bug by marking st_union as a
non-duplicate-ignoring aggregate.

Fixes #111556

Release note (bug fix): Queries with the st_union aggregate function
could produce incorrect results in some cases due to the query optimizer
performing invalid optimizations. This bug has been present since the
st_union function was introduced in version 20.2.0.

---

Release justification: Low-risk bug fix that fixes incorrect query
results when using the st_union aggregate function.

[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 

[michae2]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball, @rafiss, and @yuzefovich)

[DrewKimball]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @rafiss and @yuzefovich)