Ensure datetimepicker widget overlay shows over modals

- Update to the exact same z-index as the modal
- Fixes #3604

client/scss/overrides/_vendor.datetimepicker.scss

@@ -8,7 +8,7 @@
   padding-inline-start: 0;
   padding-top: 2px;
   position: absolute;
-  z-index: 5;
+  z-index: 500;
   display: none;
 
   .w-dialog & {