Fix #3347943: Better variants to HTML class conversions

Closes #3347943

templates/patterns/accordion/pattern-accordion.html.twig

-{% if variant != '' and variant|lower != 'default' %}
-  {% set variants = variant|split('__') %}
-  {% for variant in variants %}
-    {% set attributes = attributes.addClass('accordion-' ~ variant|lower|replace({'_': '-'})) %}
-  {% endfor %}
+{% if variant and variant|lower != 'default' %}
+  {% set attributes = attributes.addClass('accordion-' ~ variant|lower|replace({'_': '-'})) %}
 {% endif %}
 {% set attributes = attributes.addClass('accordion') %}
 {% set attributes = accordion_id ? attributes.setAttribute('id', accordion_id) : attributes %}