Update methods, and make placeholders persist

Previously, when you reloaded placeholderapi, the DXL expansion would get unloaded. This aims to prevent it from unloading whenever PAPI was reloaded. It also updates the onRequest() method

core/src/main/java/de/erethon/dungeonsxl/util/PlaceholderUtil.java

@@ -56,9 +56,14 @@ public class PlaceholderUtil extends PlaceholderExpansion {
     public String getVersion() {
         return plugin.getDescription().getVersion();
     }
+    
+    @Override
+    public boolean persist() {
+        return true;
+    }
 
     @Override
-    public String onPlaceholderRequest(Player player, String identifier) {
+    public String onRequest(OfflinePlayer player, String identifier) {
         if (player == null) {
             return "";
         }