fix(bot): only allow proxying in known-supported channel types

This is so that new channel types added by Discord (that may or may not
support the features we need for proxying to work) don't throw piles of
error codes at users when they try to proxy.

Myriad/Types/Channel.cs

@@ -14,7 +14,10 @@ public record Channel
         GuildNewsThread = 10,
         GuildPublicThread = 11,
         GuildPrivateThread = 12,
-        GuildStageVoice = 13
+        GuildStageVoice = 13,
+        GuildDirectory = 14,
+        GuildForum = 15,
+        GuildMedia = 16,
     }
 
     public ulong Id { get; init; }