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/ChatConfig.java

@@ -157,5 +157,9 @@ public class ChatConfig {
                     .thenText("] ");
         }
 
+        private PandaTheme() {}
+
     }
+
+    private ChatConfig() {}
 }