ICU-715 Change ExperimentalGroupUnreadChannels setting to allow for default on/off (#8211)

i18n/en.json

@@ -4758,6 +4758,10 @@
     "id": "model.config.is_valid.file_thumb_width.app_error",
     "translation": "Invalid thumbnail width for file settings.  Must be a positive number."
   },
+  {
+    "id": "model.config.is_valid.group_unread_channels.app_error",
+    "translation": "Invalid group unread channels for service settings. Must be 'disabled', 'default_on', or 'default_off'."
+  },
   {
     "id": "model.config.is_valid.image_proxy_type.app_error",
     "translation": "Invalid image proxy type for service settings."