Fix compilation in Eclipse (#21606)

* Fix compilation in Eclipse

I'm not sure what the bug is, but ecj doesn't like this expression
unless the type is set explicitly.

* Add comment explaining why no diamond operator

core/src/main/java/org/elasticsearch/action/support/replication/TransportReplicationAction.java

@@ -1018,7 +1018,8 @@ public abstract class TransportReplicationAction<
             }
             transportService.sendRequest(node, transportReplicaAction,
                 new ConcreteShardRequest<>(request, replica.allocationId().getId()), transportOptions,
-                new ActionListenerResponseHandler<>(listener, ReplicaResponse::new));
+                // Eclipse can't handle when this is <> so we specify the type here.
+                new ActionListenerResponseHandler<ReplicaResponse>(listener, ReplicaResponse::new));
         }
 
         @Override