SPIGOT-547: Increase the max plugin message channel name to 20

This matches up with the protocol

By: Thinkofdeath <thinkofdeath@spigotmc.org>

paper-api/src/main/java/org/bukkit/plugin/messaging/Messenger.java

@@ -18,7 +18,7 @@ public interface Messenger {
     /**
      * Represents the largest size that a Plugin Channel may be.
      */
-    public static final int MAX_CHANNEL_SIZE = 16;
+    public static final int MAX_CHANNEL_SIZE = 20;
 
     /**
      * Checks if the specified channel is a reserved name.