Fix testPendingTasksWithClusterNotRecoveredBlock

In 7.x we cannot start a new master-eligible node before the cluster has formed
since we first try and update minimum_master_nodes and this is blocked. This
commit changes the test to start a data-only node so that no such adjustment is
necessary.

Relates #44685

server/src/test/java/org/elasticsearch/action/admin/cluster/tasks/PendingTasksBlocksIT.java

@@ -91,7 +91,7 @@ public class PendingTasksBlocksIT extends ESIntegTestCase {
         assertNotNull(client().admin().cluster().preparePendingClusterTasks().get().getPendingTasks());
 
         // starting one more node allows the cluster to recover
-        internalCluster().startNode();
+        internalCluster().startDataOnlyNode(); // cannot update minimum_master_nodes before the cluster has formed
         ensureGreen();
     }