fix(collapsible-panel): remove z-index (#805)

Author: montezume

src/components/panels/collapsible-panel/collapsible-panel.styles.js

@@ -53,7 +53,6 @@ const getHeaderContainerStyles = ({ isDisabled, isOpen, isSticky, theme }) => {
       css`
         position: sticky;
         top: 0;
-        z-index: 1;
         border-top-right-radius: ${vars.borderRadius6};
         border-top-left-radius: ${vars.borderRadius6};
       `,