Alert : Added z-index for close button (#31826)

Co-authored-by: Ankit Mishra <ankitmishra@Ankit-MacBook-Pro.local>
Co-authored-by: Martijn Cuppens <martijn.cuppens@gmail.com>

scss/_alert.scss

@@ -34,6 +34,7 @@
     position: absolute;
     top: 0;
     right: 0;
+    z-index: $stretched-link-z-index + 1;
     padding: $alert-padding-y * 1.25 $alert-padding-x;
   }
 }