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-paper-permissions/src/main/java/fr/pandacube/lib/paper/permissions/PandalibPaperPermissions.java

@@ -82,6 +82,12 @@ public class PandalibPaperPermissions implements Listener {
 		}
 	}
 
+
+	/**
+	 * Creates a {@link PandalibPaperPermissions} instance.
+	 */
+	private PandalibPaperPermissions() {}
+
 	/**
 	 * Player login event handler.
 	 * @param event the event.