Update Subnav to use the new nav data structure (#11324)

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### WHY are these changes introduced?

[Fixes #0000](#11322) <!-- link
to issue if one exists -->

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  Context about the problem that’s being addressed.
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### WHAT is this pull request doing?
The `nav.json` data was updated and the `Subnav` did not update to match
the new structure, so we were not getting the data properly. This is
updating to match by passing in `nav.children`



https://github.com/Shopify/polaris/assets/4642404/ab987492-3888-42c3-9d46-a7c2e7f9168f


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### How to 🎩

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### 🎩 checklist

- [ ] Tested a
[snapshot](https://github.com/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [ ] Tested on
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- [ ] 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

polaris.shopify.com/src/components/Subnav/Subnav.tsx

@@ -81,15 +81,15 @@ function getNavItems(path: string): {[key: string]: NavItem} | undefined {
   const paths = path.split('/').filter((segment) => segment);
 
   const navItemPath = paths.join('.children.');
-  const currentNavItem = getObjectValue<NavItem>(nav, navItemPath);
+  const currentNavItem = getObjectValue<NavItem>(nav.children, navItemPath);
 
   const isOverviewPage = currentNavItem?.children !== undefined;
 
   // Get the parent nav item if we're on a subpage
   const parentItemPath = paths.slice(0, -1).join('.children.');
   const parentNavItem = isOverviewPage
     ? currentNavItem
-    : getObjectValue<NavItem>(nav, parentItemPath);
+    : getObjectValue<NavItem>(nav.children, parentItemPath);
 
   // Return if we're on a page that doesn't have a subnav
   if (!parentNavItem) return;