release-21.1: kv: add to replicaGCQueue in replicaMsgAppDropper, not gcQueue

[blathers-crl]

Backport 1/1 commits from #74073 on behalf of @nvanbenschoten.

/cc @cockroachdb/release

---

Fixes #73838.

This commit is the first of the three "next steps" identified in #73838. It fixes a case where we were accidentally adding a replica to the wrong queue. When dropping a MsgApp in maybeDropMsgApp, we want to GC the replica on the LHS of the split if it has been removed from its range. However, we were instead passing it to the MVCC GC queue, which was both irrelevant and a no-op because the LHS was not the leaseholder.

It's possible that we have seen the effects of this in roachtests like splits/largerange. This but could have delayed a snapshot to the RHS of a split for up to maxDelaySplitTriggerTicks * 200ms = 20s in some rare cases. We've seen the logs corresponding to this issue in a few tests over the past year: https://github.com/cockroachdb/cockroach/issues?q=is%3Aissue+%22would+have+dropped+incoming+MsgApp+to+wait+for+split+trigger%22+is%3Aclosed.

---

Release justification: bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is