Fix Javadoc warnings due to Java 21 update (+ some other warnings)

The default implicit constructor must also have a doc comment, so I have to make it explicit and either properly restrict the visibility of the constructor, or actually document it.

pandalib-ws-server/src/main/java/fr/pandacube/lib/ws/server/AbstractServerWS.java

@@ -14,6 +14,11 @@ import java.time.Duration;
 public abstract class AbstractServerWS extends WebSocketAdapter implements AbstractWS {
 
 	private boolean isClosed = false;
+
+	/**
+	 * Creates a Websocket server endpoint.
+	 */
+	public AbstractServerWS() {}
 	
 	@Override
 	public final void onWebSocketConnect(Session session)