release-23.2: kvserver: acquire Replica.mu when returning reproposal error

[blathers-crl]

Backport 1/1 commits from #117801 on behalf of @erikgrinaker.

/cc @cockroachdb/release

---

Discovered while working on #117612. Looks like it's been there since #42939.

This is a rare error, and these fields are unlikely to change while we're holding the raft mutex, so seems very unlikely to have caused any problems -- I could be convinced we shouldn't backport this, on the off-chance I've missed a potential deadlock.

Epic: none
Release note: None

---

Release justification: fixes a rare race condition.

[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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Also, please add a brief release justification to the body of your PR to justify this
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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?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[stevendanna]

Seems reasonable to backport to me. While you mention there is some chance of this introducing a deadlock, I think it makes sense to rely on our tests here rather than avoiding making a clear correctness change.