Migrate spinners in explore groups partial

Changelog: changed

app/views/explore/groups/_groups.html.haml

 .js-groups-list-holder
   #js-groups-tree{ data: { hide_projects: 'true', endpoint: explore_groups_path(format: :json), path: explore_groups_path, form_sel: 'form#group-filter-form', filter_sel: '.js-groups-list-filter', holder_sel: '.js-groups-list-holder', dropdown_sel: '.js-group-filter-dropdown-wrap' } }
-    .loading-container.text-center.prepend-top-20
-      .gl-spinner.gl-spinner-md
+    = gl_loading_icon(size: 'md', css_class: 'gl-mt-6')