Prevent attempts to send greeting message if the set channel is an Object

- Class `GreetingChannel` under class `MemberAdminSettings` no longer accepts Objects as a value.

src/modules/member_admin/settings.py

@@ -55,6 +55,7 @@ class MemberAdminSettings(SettingGroup):
 
         _model = CoreData.Guild
         _column = CoreData.Guild.greeting_channel.name
+        _allow_object = False
 
         @property
         def update_message(self) -> str: