Update iOS add-to-app "flutter_post_install" instructions to indicate available version number

[jmagman]

Page URL

https://docs.flutter.dev/development/add-to-app/ios/project-setup#option-a---embed-with-cocoapods-and-the-flutter-sdk

Page source

No response

Describe the problem

When https://github.com/flutter/flutter/pull/101943 makes it to a tagged release, update The flutter_post_install method (recently added to Flutter), line to instead indicate the version in which it's available.

Introduced in https://github.com/flutter/website/pull/7032#issuecomment-1100241706.

Expected fix

No response

Additional context

No response