Add final

Signed-off-by: Craig Perkins <[email protected]>

server/src/main/java/org/opensearch/identity/noop/NoopPluginSubject.java

@@ -18,7 +18,7 @@
 import java.util.concurrent.Callable;
 
 public class NoopPluginSubject extends AbstractSubject {
-    private ThreadPool threadPool;
+    private final ThreadPool threadPool;
 
     public NoopPluginSubject(ThreadPool threadPool) {
         super();