staging-v24.3.13: admission: increase file count based overload threshold

[blathers-crl]

Backport 1/1 commits from #145919 on behalf of @blathers-crl[bot].

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Backport 1/1 commits from #138699 on behalf of @sumeerbhola.

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The current value of 1000 was too inconsistent with Pebble's compaction scoring, in that compaction scoring had 1 sub-level == 500 L0 files, and admission control had 1 sub-level == 50 L0 files. The value is increased to 4000, so 1 sub-level == 200 L0 files. There is a long code comment elaborating on this.

Informs #138655

Epic: none

Release note (ops change): The default value of cluster setting admission.l0_file_count_overload_threshold is changed to 4000.

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Release justification: Addresses insufficient tokens under high write load and WAL failover.

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Release justification:

[blathers-crl]

Thanks for opening a backport.

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[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?

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[cockroach-teamcity]

This change is