Center modal dialog vertically

netbox/templates/inc/htmx_modal.html

@@ -1,5 +1,5 @@
 <div class="modal fade" id="htmx-modal" tabindex="-1" aria-hidden="true">
-  <div class="modal-dialog">
+  <div class="modal-dialog modal-dialog-centered">
     <div class="modal-content" id="htmx-modal-content">
       {# Dynamic content goes here #}
     </div>