v4: Add z-index to .custom-check to fix custom checks in CSS columns (#30810)

For some reason, custom checks in CSS columns need a new stacking order to maintain visibility. This PR adds `z-index: 1` to trigger that new stacking order. Since v5's forms are being redone entirely, and we're dropping IE10/11 there, I think this is totally fine to ship with v4 only. Will need to check v5's forms it in Edge though.

Fixes #29607

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Co-authored-by: XhmikosR <xhmikosr@gmail.com>

scss/_custom-forms.scss

@@ -9,6 +9,7 @@
 
 .custom-control {
   position: relative;
+  z-index: 1;
   display: block;
   min-height: $font-size-base * $line-height-base;
   padding-left: $custom-control-gutter + $custom-control-indicator-size;