release-24.2: crosscluster/physical: use correct lag replan cluster setting

[blathers-crl]

Backport 1/1 commits from #131038 on behalf of @msbutler.

/cc @cockroachdb/release

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Previously the stream_replication.replan_flow_frequency setting determined the frequency that the frontier processor checked for lagging nodes, not the stream_replication.lag_check_frequency setting, as intended. The latter didn't control anything. This patch fixes this.

Epic: none
Release note: none

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

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Also, please add a brief release justification to the body of your PR to justify this
backport.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is