Finalize the ServerDisconnectEvent target field.

There is no good reason for it to be mutable in the first place, considering that the event instance is not used after the method is called.

api/src/main/java/net/md_5/bungee/api/event/ServerDisconnectEvent.java

@@ -25,5 +25,5 @@ public class ServerDisconnectEvent extends Event
      * Server the player is disconnecting from.
      */
     @NonNull
-    private ServerInfo target;
+    private final ServerInfo target;
 }