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-chat/src/main/java/fr/pandacube/lib/chat/ChatUtil.java

@@ -657,5 +657,6 @@ public class ChatUtil {
         return str;
     }
 
+    private ChatUtil() {}
 
 }