fix(ui): increase z-index of ReactSelect (#8735)

Fixes https://github.com/payloadcms/payload/issues/8728

Before:
<img width="698" alt="Screenshot 2024-10-16 at 15 28 55"
src="https://github.com/user-attachments/assets/eee45448-5e97-4c2a-bbe3-727c41ed9b08">
After:
<img width="509" alt="Screenshot 2024-10-16 at 15 29 27"
src="https://github.com/user-attachments/assets/7e0a2af6-71be-41e7-ad84-4ae3bcece9b6">

packages/ui/src/elements/ReactSelect/index.scss

@@ -6,6 +6,9 @@
   }
 
   .react-select {
+    &:focus-within {
+      z-index: 3;
+    }
     .rs__control {
       @include formInput;
       height: auto;