Fix StageChannel.last_message_id always being None

Author: owocado

discord/state.py

@@ -615,7 +615,7 @@ def parse_message_create(self, data: gw.MessageCreateEvent) -> None:
         if self._messages is not None:
             self._messages.append(message)
         # we ensure that the channel is either a TextChannel, VoiceChannel, or Thread
-        if channel and channel.__class__ in (TextChannel, VoiceChannel, Thread):
+        if channel and channel.__class__ in (TextChannel, VoiceChannel, Thread, StageChannel):
             channel.last_message_id = message.id  # type: ignore
 
     def parse_message_delete(self, data: gw.MessageDeleteEvent) -> None: