release-20.2: kvserver: prevent StoreRebalancer from downreplicating

When the replication factor is lowered and the StoreRebalancer
attempts a rebalance, it will accidentally perform a downreplication.
Since it wasn't ever supposed to do that, the downreplication is
pretty haphazard and doesn't safeguard quorum in the same way
that a "proper" downreplication likely would. Prevent if from
changing the number of voters and non-voters to avoid this issue.

Annoyingly, I [knew] about this problem, but instead of fixing it at the
source - as this commit does - I added a lower- level check that could
then not be backported to release-20.2, where we are now seeing this
problem.

[knew]: cockroachdb#54444 (comment)

Release note: None

pkg/kv/kvserver/store_rebalancer.go

@@ -506,6 +506,12 @@ func (sr *StoreRebalancer) chooseReplicaToRebalance(
 		targetReplicas := make([]roachpb.ReplicaDescriptor, 0, desiredReplicas)
 		currentReplicas := desc.Replicas().All()
 
+		if cur := len(desc.Replicas().Voters()); desiredReplicas != cur {
+			log.VEventf(ctx, 3, "cannot change number of voters from %d to %d for r%d",
+				cur, desiredReplicas, desc.RangeID)
+			continue
+		}
+
 		// Check the range's existing diversity score, since we want to ensure we
 		// don't hurt locality diversity just to improve QPS.
 		curDiversity := rangeDiversityScore(