fix: Replace double-checked locking idiom with initialization-on-dema…

…nd holder idiom

- https://rules.sonarsource.com/java/tag/multi-threading/RSPEC-2168
- https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom

kubernetes-client/src/main/java/io/fabric8/kubernetes/client/internal/readiness/Readiness.java

@@ -49,15 +49,12 @@ public class Readiness {
   protected static final String READINESS_APPLICABLE_RESOURCES =
     "Node, Deployment, ReplicaSet, StatefulSet, Pod, ReplicationController";
 
-  private static Readiness instance;
+  private static class ReadinessHolder {
+    public static final Readiness INSTANCE = new Readiness();
+  }
 
   public static Readiness getInstance() {
-    if (instance == null) {
-      synchronized (Readiness.class) {
-        instance = new Readiness();
-      }
-    }
-    return instance;
+    return ReadinessHolder.INSTANCE;
   }
 
   /**

openshift-client/src/main/java/io/fabric8/openshift/client/internal/readiness/OpenShiftReadiness.java

@@ -27,15 +27,12 @@ public class OpenShiftReadiness extends Readiness {
   private static final String OPENSHIFT_READINESS_APPLICABLE_RESOURCES = READINESS_APPLICABLE_RESOURCES +
     ", " + "DeploymentConfig";
 
-  private static OpenShiftReadiness instance;
+  private static class OpenShiftReadinessHolder {
+    public static final OpenShiftReadiness INSTANCE = new OpenShiftReadiness();
+  }
 
   public static OpenShiftReadiness getInstance() {
-    if (instance == null) {
-      synchronized (OpenShiftReadiness.class) {
-        instance = new OpenShiftReadiness();
-      }
-    }
-    return instance;
+    return OpenShiftReadinessHolder.INSTANCE;
   }
 
   @Override