Updated the <textarea> template.

templates/form/textarea.html.twig

@@ -0,0 +1,21 @@
+{#
+/**
+ * @file
+ * Theme override for a 'textarea' #type form element.
+ *
+ * Available variables
+ * - wrapper_attributes: A list of HTML attributes for the wrapper element.
+ * - attributes: A list of HTML attributes for the textarea element.
+ * - resizable: An indicator for whether the textarea is resizable.
+ * - required: An indicator for whether the textarea is required.
+ * - value: The textarea content.
+ *
+ * @see template_preprocess_textarea()
+ */
+#}
+
+{% set classes = ['au-text-input', 'au-text-input--block'] %}
+
+<div{{ wrapper_attributes }}>
+  <textarea{{ attributes.addClass(classes) }}>{{ value }}</textarea>
+</div>