[B] Minecraft allows a length of 20 for the Channel Name not 16

The Issue:

Bukkit throws a Exception because the Channel Name is too long when using a Plugin Message Channel of the length of 18.

Justification for this PR:

Set the Bukkit allowed max Channel length to 20 to match up the latest NMS Code for CustomPayloads

Testing Results and Materials:

This PR does only change the limit. It works fine now with a 18 character long Plugin Message Channel

JIRA Ticket:

BUKKIT-5472: https://bukkit.atlassian.net/browse/BUKKIT-5472