Replace all visually-hidden classes

These have been replaced:

https://design-system.service.gov.uk/get-started/updating-your-code/#helper-class-names

app/templates/views/dashboard/dashboard.html

@@ -14,7 +14,7 @@
 
   <div class="dashboard">
 
-    <h1 class="visuallyhidden">Dashboard</h1>
+    <h1 class="govuk-visually-hidden">Dashboard</h1>
     {% if current_user.has_permissions('manage_templates') and not current_service.all_templates %}
       {% include 'views/dashboard/write-first-messages.html' %}
     {% endif %}