Manually migrate z-index custom properties from v10 to v11 (#8425)

### WHY are these changes introduced?

Part of #8405 <!-- link to issue if one exists -->

### WHAT is this pull request doing?
Finds and replaces any instances of legacy `z-index` custom properties
in non SCSS files with the repalcement map from
[`v11-styles-replace-custom-property-zindex`](#8272)

.changeset/hungry-carpets-lay.md

@@ -0,0 +1,6 @@
+---
+'@shopify/polaris': minor
+'polaris.shopify.com': minor
+---
+
+Manually migrated `z-index` custom properties from `v10` to `v11`

polaris-react/src/components/Sheet/Sheet.stories.tsx

@@ -345,7 +345,7 @@ export function WithSearchableListbox() {
       style={{
         padding: 'var(--p-space-4) var(--p-space-2)',
         position: 'sticky',
-        zIndex: 'var(--p-z-12)',
+        zIndex: 'var(--p-z-index-12)',
         width: '100%',
         background: 'var(--p-surface)',
       }}

polaris.shopify.com/pages/examples/sheet-with-searchable-listbox.tsx

@@ -194,7 +194,7 @@ function SheetWithSearchableListboxExample() {
       style={{
         padding: 'var(--p-space-4) var(--p-space-2)',
         position: 'sticky',
-        zIndex: 'var(--p-z-12)',
+        zIndex: 'var(--p-z-index-12)',
         width: '100%',
         background: 'var(--p-surface)',
       }}