[BS5] add whats new modal after update

data/web/templates/modals/admin.twig

@@ -261,3 +261,19 @@
     </div>
   </div>
 </div><!-- priv key modal -->
+<!-- whats new modal -->
+<div class="modal fade show" id="showWhatsNew" tabindex="-1" role="dialog" aria-hidden="true">
+  <div class="modal-dialog modal-lg">
+    <div class="modal-content">
+      <div class="modal-header">
+        <h5 class="modal-title">What's new?</h5>
+        <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
+      </div>
+      <div class="modal-body d-flex flex-column mt-2 p-4">
+        <div class="spinner-border mx-auto" role="status">
+          <span class="visually-hidden">Loading...</span>
+        </div>
+      </div>
+    </div>
+  </div>
+</div><!-- whats new modal -->