in_app_purchases codelab: verifyReceipt is deprecated

### Is there an existing issue for this?

- [X] I have searched the [existing issues](https://github.com/flutter/flutter/issues)
- [X] I have read the [guide to filing a bug](https://flutter.dev/docs/resources/bug-reports)

### Use case

I couldn't find the correct issue type, but the codelab "report a mistake" link brought me here. The issue relates to this page:

https://codelabs.developers.google.com/codelabs/flutter-in-app-purchases#9

The iTunes `verifyReceipt` endpoint [is now deprecated](https://developer.apple.com/documentation/storekit/in-app_purchase/original_api_for_in-app_purchase/validating_receipts_with_the_app_store?language=objc).

By the way, some of the backend setup instructions are also out of date in the codelab:

https://codelabs.developers.google.com/codelabs/flutter-in-app-purchases#8

For example, setting up Google Play API access is no longer done through the Play console, it is done through the cloud console.

### Proposal

N/A