Fix AckClusterUpdateSettingsIT.testClusterUpdateSettingsAcknowledgement() after changes in #14259

Closes #14278

core/src/test/java/org/elasticsearch/cluster/ack/AckClusterUpdateSettingsIT.java

@@ -29,6 +29,7 @@ import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.routing.IndexRoutingTable;
 import org.elasticsearch.cluster.routing.IndexShardRoutingTable;
 import org.elasticsearch.cluster.routing.ShardRouting;
+import org.elasticsearch.cluster.routing.allocation.decider.ConcurrentRebalanceAllocationDecider;
 import org.elasticsearch.cluster.routing.allocation.decider.ThrottlingAllocationDecider;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.discovery.DiscoverySettings;
@@ -50,6 +51,7 @@ public class AckClusterUpdateSettingsIT extends ESIntegTestCase {
                 //make sure that enough concurrent reroutes can happen at the same time
                 //we have a minimum of 2 nodes, and a maximum of 10 shards, thus 5 should be enough
                 .put(ThrottlingAllocationDecider.CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_RECOVERIES, 5)
+                .put(ConcurrentRebalanceAllocationDecider.CLUSTER_ROUTING_ALLOCATION_CLUSTER_CONCURRENT_REBALANCE, 10)
                 .build();
     }