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-bungee/src/main/java/fr/pandacube/lib/bungee/PandaLibBungee.java

@@ -43,4 +43,7 @@ public class PandaLibBungee {
     public static Plugin getPlugin() {
         return plugin;
     }
+
+    private PandaLibBungee() {}
+
 }