kvpb: mark subsume as isWrite

Now that subsume is required to force flush all replication streams up
to the lease applied index it froze the range at, it also needs to be
marked as a write in order to pass through the replication code path,
which update the range controller force flush index.

Part of: #132614
Release note: None

pkg/kv/kvpb/api.go

@@ -2113,7 +2113,7 @@ func (r *RefreshRangeRequest) flags() flag {
 	return isRead | isTxn | isRange | updatesTSCache
 }
 
-func (*SubsumeRequest) flags() flag    { return isRead | isAlone | updatesTSCache }
+func (*SubsumeRequest) flags() flag    { return isWrite | isAlone | updatesTSCache }
 func (*RangeStatsRequest) flags() flag { return isRead }
 func (*QueryResolvedTimestampRequest) flags() flag {
 	return isRead | isRange | requiresClosedTSOlderThanStorageSnapshot