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>