release-23.2: doc: make sql.index_recommendation.drop_unused_duration public

[blathers-crl]

Backport 1/1 commits from #118676 on behalf of @maryliag.

/cc @cockroachdb/release

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Make the cluster setting sql.index_recommendation.drop_unused_duration public.

Fixes #104734

Release note (sql change): Make the cluster setting sql.index_recommendation.drop_unused_duration public for docs.

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Release justification: doc update

[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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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?

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[cockroach-teamcity]

This change is