SPIGOT-7971: NotSerializableException on serialize CraftUseCooldownComponent

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/components/CraftUseCooldownComponent.java

@@ -38,7 +38,7 @@ public final class CraftUseCooldownComponent implements UseCooldownComponent {
         Map<String, Object> result = new LinkedHashMap<>();
         result.put("seconds", getCooldownSeconds());
         if (getCooldownGroup() != null) {
-            result.put("cooldown-group", getCooldownGroup());
+            result.put("cooldown-group", getCooldownGroup().toString());
         }
 
         return result;