release-21.2: streamingccl: skip TestSinklessReplicationClient on 21.2

[blathers-crl[bot]]

Backport 1/1 commits from #85342 on behalf of @gh-casper.

/cc @cockroachdb/release

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Skip this flaky test on release 21.2 branch as tenant streaming is not released in 21.2.

Release note (bug fix): skip TestSinklessReplicationClient on release 21.2

Closes: https://github.com/cockroachdb/cockroach/issues/84119

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Release justification: Test-only change to non-production code.

[blathers-crl[bot]]

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 

[HonoreDB]

Git seems to think there's nothing to backport here.

[github-actions[bot]]

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

[rafiss]

closing since this is an empty diff