Disallow changing type when updating a channel (#13387)

i18n/en.json

@@ -375,6 +375,10 @@
     "id": "api.channel.update_channel.tried.app_error",
     "translation": "Tried to perform an invalid update of the default channel {{.Channel}}"
   },
+  {
+    "id": "api.channel.update_channel.typechange.app_error",
+    "translation": "Channel type cannot be updated"
+  },
   {
     "id": "api.channel.update_channel_member_roles.changing_guest_role.app_error",
     "translation": "Invalid channel member update: You can't add or remove the guest role manually"