Increase timeout in testCleanUpShardFollowTasksForDeletedIndices (#64562)

If the deleted index has N shards, then ShardFollowTaskCleaner can send 
N*(N-1)/2 requests to remove N shard-follow tasks. I think that's fine
as the implementation is straightforward. The test failed when the
deleted index has 8 shards. This commit increases the timeout in the
test.

Closes #64311

x-pack/plugin/ccr/src/internalClusterTest/java/org/elasticsearch/xpack/ccr/IndexFollowingIT.java

@@ -1389,7 +1389,7 @@ public class IndexFollowingIT extends CcrIntegTestCase {
             String action = ShardFollowTask.NAME + "[c]";
             ListTasksResponse listTasksResponse = followerClient().admin().cluster().prepareListTasks().setActions(action).get();
             assertThat(listTasksResponse.getTasks(), hasSize(0));
-        });
+        }, 60, TimeUnit.SECONDS);
         ensureNoCcrTasks();
     }