release-22.1: settings: retire kv.rangefeed.separated_intent_scan.enabled

[blathers-crl]

Backport 1/1 commits from #78246 on behalf of @stevendanna.

/cc @cockroachdb/release

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This setting was added in a 21.2 backport

#72315

but was never added to master:

#71295

So from the perspective of someone who upgrades from 21.2, it is now a
retired setting.

Release justification: Low-risk bug fix

Release note: None

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[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 

[github-actions]

Reminder: it has been 3 weeks please merge or close your backport!

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error getting backport branch release-pr-activity: GET https://api.github.com/repos/cockroachdb/cockroach/branches/release-pr-activity: 404 Branch not found []

Backport to branch pr-activity failed. See errors above.

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