release-24.1: changefeedccl: fix schema change boundary race condition

[blathers-crl]

Backport 1/1 commits from #137567 on behalf of @andyyang890.

/cc @cockroachdb/release

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This patch fixes an issue where a schema change could incorrectly
cause a changefeed to fail with an assertion error like received boundary timestamp ... of type ... before reaching existing boundary of type ... due to a race condition between aggregators sending
resolved spans for multiple schema change descriptor updates to
the change frontier.

Fixes #136847

Release note (bug fix): An issue where a schema change could
incorrectly cause a changefeed to fail with an assertion error
like received boundary timestamp ... of type ... before reaching existing boundary of type ... has now been fixed.

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Release justification: low-risk, high-value bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[cockroach-teamcity]

This change is