Fix loading spinner animation.

This broken when upgrading bootstrap.

See https://getbootstrap.com/docs/5.3/components/spinners/#border-spinner
vs https://getbootstrap.com/docs/4.6/components/spinners/#border-spinner

Author: meisterT

webapp/templates/public/team_submissions.html.twig

@@ -14,7 +14,7 @@
         data-problem-id="{{ problem.externalId }}"
         data-submissions-url="{{ path('public_submissions_data_cell', {teamId: team.externalid, problemId: problem.externalId}) }}">
         <div class="spinner-border" role="status">
-            <span class="sr-only">Loading...</span>
+            <span class="visually-hidden">Loading...</span>
         </div>
         {% include 'public/partials/submission_list_template.html.twig' %}
     </div>

webapp/templates/public/team_submissions_modal.html.twig

@@ -10,6 +10,6 @@
 
 {% block content %}
     <div class="spinner-border" role="status">
-        <span class="sr-only">Loading...</span>
+        <span class="visually-hidden">Loading...</span>
     </div>
 {% endblock %}