fix: locking on FeatureFlags instead of VaadinContext (#13962) (#14741)…

… (#14862)

Only lock on FeatureFlags when checking for wrapper and initializing

Fixes #13962

Co-authored-by: marcin <marcin@vaadin.com>

flow-server/src/main/java/com/vaadin/experimental/FeatureFlags.java

@@ -141,7 +141,7 @@ public static FeatureFlags get(final VaadinContext context) {
         assert context != null;
 
         FeatureFlagsWrapper attribute;
-        synchronized (context) {
+        synchronized (FeatureFlags.class) {
             attribute = context.getAttribute(FeatureFlagsWrapper.class);
 
             if (attribute == null) {