release-24.1: changefeedccl: fix panics when min_checkpoint_frequency is 0

[blathers-crl]

Backport 1/1 commits from #125317 on behalf of @wenyihu6.

/cc @cockroachdb/release

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Previously, we introduced the cluster setting changefeed.aggregator.flush_jitter
to apply some jitter to the aggregator flush frequency. This was done to reduce
the load on the coordinator by spreading out aggregator flush times. However,
that PR did not account for the case where min_checkpoint_frequency is set to
zero, which could cause panics due to rand.Int63(0). This patch fixes the issue
by adding a check for when the frequency is zero, ensuring that jitter is not
applied in such cases.

Fixes: #125312

Release note (bug fix): Fixed a bug in v24.1, v23.2, and v23.1 where using
changefeed.aggregator.flush_jitter with min_checkpoint_frequency set to zero
could cause panics.

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Release justification: (low risk bug fix) Fixed a bug in v24.1, v23.2, and v23.1 where using
changefeed.aggregator.flush_jitter with min_checkpoint_frequency set to zero
could cause panics.

[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.
• Backports should not break backwards-compatibility.
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  here).
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If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[wenyihu6]

@stevendanna Do you mind taking a look at this when you have a moment? Hoping to backport the fix and close the support issue.

[wenyihu6]

The last push was to resolve merge conflicts. (Couldn't squash and didn't want to open a new custom backport)