Use standard Guava annotation

api/src/main/java/com/velocitypowered/api/annotations/UnstableApi.java

@@ -1,14 +0,0 @@
-package com.velocitypowered.api.annotations;
-
-import java.lang.annotation.ElementType;
-import java.lang.annotation.Target;
-
-/**
- * Marks unstable API interfaces that are still maturing. These interfaces may change drastically
- * between minor releases of Velocity, and it is not guaranteed that the APIs marked with this
- * annotation will be stable over time.
- */
-@Target({ ElementType.METHOD, ElementType.TYPE, ElementType.PACKAGE })
-public @interface UnstableApi {
-
-}

api/src/main/java/com/velocitypowered/api/event/command/PlayerAvailableCommandsEvent.java

@@ -2,15 +2,15 @@ package com.velocitypowered.api.event.command;
 
 import static com.google.common.base.Preconditions.checkNotNull;
 
+import com.google.common.annotations.Beta;
 import com.mojang.brigadier.tree.RootCommandNode;
-import com.velocitypowered.api.annotations.UnstableApi;
 import com.velocitypowered.api.proxy.Player;
 
 /**
  * Allows plugins to modify the packet indicating commands available on the server to a
  * Minecraft 1.13+ client.
  */
-@UnstableApi
+@Beta
 public class PlayerAvailableCommandsEvent {
 
   private final Player player;