Fix nonsensical deprecation for specifying listener priority (#1491)

* Fix nonsensical deprecation for specifying listener priority

* Fix checkstyle error

Author: Vankka

api/src/main/java/com/velocitypowered/api/event/PostOrder.java

@@ -12,6 +12,14 @@
  */
 public enum PostOrder {
 
-  FIRST, EARLY, NORMAL, LATE, LAST, CUSTOM
+  FIRST, EARLY, NORMAL, LATE, LAST,
+
+  /**
+   * Previously used to specify that {@link Subscribe#priority()} should be used.
+   *
+   * @deprecated No longer required, you only need to specify {@link Subscribe#priority()}.
+   */
+  @Deprecated
+  CUSTOM
 
 }

api/src/main/java/com/velocitypowered/api/event/Subscribe.java

@@ -32,12 +32,9 @@
    * The priority of this event handler. Priorities are used to determine the order in which event
    * handlers are called. The higher the priority, the earlier the event handler will be called.
    *
-   * <p>Note that due to compatibility constraints, you must specify {@link PostOrder#CUSTOM}
-   * in order to use this field.</p>
-   *
    * @return the priority
    */
-  short priority() default Short.MIN_VALUE;
+  short priority() default 0;
 
   /**
    * Whether the handler must be called asynchronously. By default, all event handlers are called

proxy/src/main/java/com/velocitypowered/proxy/event/VelocityEventManager.java

@@ -350,8 +350,9 @@ private void collectMethods(final Class<?> targetClass,
         asyncType = AsyncType.ALWAYS;
       }
 
+      // The default value of 0 will fall back to PostOrder, the default PostOrder (NORMAL) is also 0
       final short order;
-      if (subscribe.order() == PostOrder.CUSTOM) {
+      if (subscribe.priority() != 0) {
         order = subscribe.priority();
       } else {
         order = (short) POST_ORDER_MAP.get(subscribe.order());