release-20.2: sql: fix cast from unknown to unknown

[blathers-crl]

Backport 1/1 commits from #68627 on behalf of @mgartner.

/cc @cockroachdb/release

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Previously, an implicit cast performed in a set operation, like UNION,
on a tuple type with an unknown inner type would cause an internal
error. The cast failed because the volatility of a cast from unknown to
unknown was not specified in the validCasts list. This commit fixes
the issue by marking a such a cast as immutable.

Fixes #68308

Release note (bug fix): A bug has been fixed that caused internal errors
with set operations, like UNION, and columns with tuple types that
contained constant NULL values. This bug was introduced in version
20.2.

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Release justification: This is a low risk bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• 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.

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