Merge branch 'navbar-z-index-higher' into 'master'

Increase the z-index of the navbar

Closes #35935

See merge request !13282

app/assets/stylesheets/framework/header.scss

@@ -26,7 +26,7 @@ header {
 
   &.navbar-gitlab {
     padding: 0 16px;
-    z-index: 400;
+    z-index: 2000;
     margin-bottom: 0;
     min-height: $header-height;
     background-color: $gray-light;