Update documentation of admin_compact_mode in system module
* If the user does not have the cookie, the default value is given by the
* system variable 'admin_compact_mode', which itself defaults to FALSE. This
* does not have a user interface to set it: it is a hidden variable which can
* be set in the settings.php file.where as this variable is config now.
config('system.site')->get('admin_compact_mode')