TL;DR: I'm adding a BWC layer in #24841 that will only be triggered in a mixed-version cluster with at least 3 nodes and an index with at least 3 shard copies (i.e. index.number_of_replicas > 1). Q: Should we change our existing qa/rolling-upgrade tests to run with 3 nodes?

Details: When a primary fails and a replica is promoted to primary, the new primary will trigger a primary-replica resync, making sure that all the replicas are realigned with the new primary. This is a transport action that will only go into 6.0, not into 5.x. As such I added a BWC layer to the transport action that it will not send these resync requests to 5.6 nodes (i.e. replicas on 5.6 nodes won't be realigned with the new 6.0 primary, same as we do today in < 6.0).