Migrate `gl-w-12` utility class to gitlab-ui

app/assets/stylesheets/pages/alert_management/list.scss

 .alert-management-list {
-  // Migrate this to gitlab-ui
-  .gl-w-12 {
-    width: $gl-spacing-scale-12;
-  }
-
   // these styles need to be deleted once GlTable component looks in GitLab same as in @gitlab/ui
   table {
     color: $gray-700;