release-25.4: spanlatch: add cluster setting for slow latch request threshold

[angeladietz]

Backport 1/1 commits from #158472 on behalf of @angeladietz.

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This adds a new cluster setting
kv.concurrency.slow_latch_request_duration which configures the threshold for logging slow latch acquisitions. Previously, logging would occur after 15 seconds, but now the default is reduced to 5s.

Informs #154271
Release note: None
Epic: None
Part of: CRDB-54843

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Release justification: low risk obs improvement to help debug slow latch requests

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that the change does not break backwards compatibility and otherwise complies with the backport policy. Include a brief release justification in the PR description explaining why the backport is appropriate. All backports must be reviewed by the TL for the owning area. While the stricter LTS policy does not yet apply, please exercise judgment and consider gating non-critical changes behind a disabled-by-default feature flag when appropriate.

[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 

[tbg]

The maybe_stress invocation is a failed build, not a failed tests.