Get rid of "this-escape" warning in NodeShutdownTasksIT (elastic#101657)

* Make test class final instead of suppressing

If this class is final, there can't be any subclasses, and therefore no subclass initialization bugs and no JDK warning.

x-pack/plugin/shutdown/src/internalClusterTest/java/org/elasticsearch/xpack/shutdown/NodeShutdownTasksIT.java

@@ -184,7 +184,7 @@ public List<NamedXContentRegistry.Entry> getNamedXContent() {
         }
     }
 
-    public static class TaskExecutor extends PersistentTasksExecutor<TestTaskParams> implements ClusterStateListener {
+    public static final class TaskExecutor extends PersistentTasksExecutor<TestTaskParams> implements ClusterStateListener {
 
         private final PersistentTasksService persistentTasksService;