Filter pre-6.0 nodes for checkpoint invariants

When checking that the global checkpoint on the primary is consistent
with the local checkpoints of the in-sync shards, we have to filter
pre-6.0 nodes from the check or the invariant will trivially trip. This
commit filters these nodes out when checking this invariant.

Relates #26666

core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointTracker.java

@@ -316,7 +316,7 @@ public class GlobalCheckpointTracker extends AbstractIndexShardComponent {
                                 .stream()
                                 .filter(cps -> cps.inSync)
                                 .mapToLong(function)
-                                .filter(v -> v != SequenceNumbers.UNASSIGNED_SEQ_NO));
+                                .filter(v -> v != SequenceNumbersService.PRE_60_NODE_CHECKPOINT && v != SequenceNumbers.UNASSIGNED_SEQ_NO));
         return value.isPresent() ? value.getAsLong() : SequenceNumbers.UNASSIGNED_SEQ_NO;
     }