release-22.2: changefeedccl: fix kafka messagetoolarge test failure

[blathers-crl]

Backport 1/1 commits from #98471 on behalf of @samiskin.

/cc @cockroachdb/release

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Fixes: #93847

This fixes the following bug in the TestChangefeedKafkaMessageTooLarge test setup:

1. The feed starts sending messages, randomly triggering a MessageTooLarge error causing a retry with a smaller batch size
2. Eventually, while the retrying process is still ongoing, all 2000 rows are successfully received by the mock kafka sink, causing assertPayloads to complete, causing the test to closeFeed and run CANCEL on the changefeed.
3. The retrying process gets stuck in sendMessage where it can't send the message to the feedCh which has been closed since the changefeed is trying to close, but it also can't exit on the mock sink's tg.done since that only closes after the feed fully closes, which requires the retrying process to end.

Release note: None

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Release justification:

[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.

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• 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?

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