[IndexTable] Fix header cutting off loading banner due to z-index (#8364

)

### WHY are these changes introduced?

Related to: #8277
In order to support some tooltip behavior the z-index changed for table
headers. Loading banners need to have their z-index moved up to not be
cut off by the header.

Example issue:
![Screenshot 2023-02-15 at 9 42 03
AM](https://user-images.githubusercontent.com/510085/219060518-a2fdffab-9263-42c0-8647-9278f8322c52.jpg)

### WHAT is this pull request doing?

This PR removes `--pc-index-table-loading-panel` and the loading banner
will now use `--p-z-2` for z-index instead.

Example with fix:
![Screenshot 2023-02-15 at 9 42 21
AM](https://user-images.githubusercontent.com/510085/219060886-8f6c5d18-e596-41b9-9e6a-1c4e42b8a728.jpg)

### 🎩 checklist

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [ ] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

.changeset/tasty-seals-collect.md

@@ -0,0 +1,6 @@
+---
+'@shopify/polaris': patch
+---
+
+Replace --pc-index-table-loading-panel with --p-z-2 to resolve loading bar overlap issue with .LoadingPanel
+Rename --pc-index-table-loading-panel to --pc-index-table-sticky-table for continued use in .StickyTable

polaris-react/src/components/IndexTable/IndexTable.scss

@@ -8,7 +8,7 @@
   --pc-index-table-sticky-cell: 31;
   --pc-index-table-scroll-bar: 35;
   --pc-index-table-bulk-actions: 36;
-  --pc-index-table-loading-panel: 37;
+  --pc-index-table-sticky-table: 37;
   // stylelint-enable
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
   position: relative;
@@ -57,7 +57,7 @@ $loading-panel-height: 53px;
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
   position: absolute;
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
-  z-index: var(--pc-index-table-loading-panel);
+  z-index: var(--p-z-2);
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
   top: 0;
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
@@ -495,7 +495,7 @@ $loading-panel-height: 53px;
   right: 0;
   visibility: hidden;
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
-  z-index: var(--pc-index-table-loading-panel);
+  z-index: var(--pc-index-table-sticky-table);
 }
 
 .StickyTableHeader {