Make SessionContextState a static nested class

Fixes a compilation error with GraalVM JDK 22.
Fixes #39846

extensions/websockets-next/server/runtime/src/main/java/io/quarkus/websockets/next/runtime/WebSocketSessionContext.java

@@ -215,7 +215,7 @@ public Event<Object> get() {
         });
     }
 
-    class SessionContextState implements ContextState {
+    static class SessionContextState implements ContextState {
 
         // Using 0 as default value enable removing an initialization
         // in the constructor, piggybacking on the default value.