Issue #3480830 by spryah, pdureau: Display local tasks as a button group

Closes #3480830

templates/overrides/system/menu-local-task.html.twig

@@ -16,12 +16,12 @@
  * @ingroup themeable
  */
 #}
-{% set variant = is_active ? 'selected' : '' %}
+{% set variant = is_active ? '' : 'tertiary' %}
 {% if is_active %}
   {% set attributes = attributes.setAttribute('aria-current', 'page') %}
 {% endif %}
-{{ pattern('tab', {
+{{ pattern('button', {
   attributes: attributes,
-  title: link['#title'],
+  label: link['#title'],
   url: link['#url'].toString(),
 }, variant) }}