Fix pre-6.0 response to unknown replication actions #25744
When sending replica requests for replication operations, we skip sending the request to pre-6.0 nodes for operations that such nodes would not be aware of (e.g., the background global checkpoint sync, or the primary/replica resync) since they would not know what to do with these requests. Yet, we simulate that we received responses from these nodes. Today, this is done by simulating that they sent us that their local checkpoint is unassigned sequence number. However, for pre-6.0 nodes we have introduced a special local checkpoint used in the global checkpoint tracker for such nodes and that is what we should use here too. This commit fixes this issue.