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() {}
+
 }

pandalib-bungee/src/main/java/fr/pandacube/lib/bungee/backup/BungeeBackupConfig.java

@@ -35,4 +35,11 @@ public class BungeeBackupConfig {
      * A list of ignored files or directory in the workdir to exclude from the backup.
      */
     public List<String> workdirIgnoreList = new ArrayList<>();
+
+    /**
+     * Creates a new {@link BungeeBackupConfig}.
+     */
+    public BungeeBackupConfig() {
+
+    }
 }