Mention route field in app router migration (vercel#70429)

Closes: vercel#59342

docs/02-app/01-building-your-application/11-upgrading/04-app-router-migration.mdx

@@ -474,6 +474,7 @@ In addition, the new `useRouter` hook has the following changes:
 - `basePath` has been removed. The alternative will not be part of `useRouter`. It has not yet been implemented.
 - `asPath` has been removed because the concept of `as` has been removed from the new router.
 - `isReady` has been removed because it is no longer necessary. During [static rendering](/docs/app/building-your-application/rendering/server-components#static-rendering-default), any component that uses the [`useSearchParams()`](/docs/app/api-reference/functions/use-search-params) hook will skip the prerendering step and instead be rendered on the client at runtime.
+- `route` has been removed. `usePathname` or `useSelectedLayoutSegments()` provide an alternative.
 
 [View the `useRouter()` API reference](/docs/app/api-reference/functions/use-router).