Merge branch '31821-flash-notifications-should-not-be-hidden-behind-other-elements' into 'master'

Increase Flash notification z-index to make it on top of the other elements

Closes #31821

See merge request gitlab-org/gitlab!17178

app/assets/stylesheets/framework/flash.scss

@@ -12,7 +12,7 @@ $notification-box-shadow-color: rgba(0, 0, 0, 0.25);
     position: sticky;
     position: -webkit-sticky;
     top: $flash-container-top;
-    z-index: 200;
+    z-index: 251;
 
     .flash-content {
       box-shadow: 0 2px 4px 0 $notification-box-shadow-color;