examples: update app-dir-i18n-routing to v15 (#72636)
## Why?
The example `app-dir-i18n-routing` needs to be updated to v15.
## How?
Run the `next-async-request-api` codemod:
```
pnpm dlx @next/codemod@latest next-async-request-api .
```
- Fixes https://github.com/vercel/next.js/issues/72630
