release-23.1.18-rc: util/mon: lose more references on Stop

[blathers-crl]

Backport 1/1 commits from #120789 on behalf of @yuzefovich.

/cc @cockroachdb/release

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Backport 1/1 commits from #120764.

/cc @cockroachdb/release

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This commit makes it so that we also now lose the references from the monitor being Stopped to its siblings to aid GC. This was omitted by mistake originally and perhaps could make GC's job harder.

Epic: None

Release note: None

Release justification: low-risk improvement.

[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