Add iOS 15 to supported platforms list

[jmagman]

Page URL

https://docs.flutter.dev/development/tools/sdk/release-notes/supported-platforms

Page source

No response

Describe the problem

Flutter infrastructure (framework and plugins) is now testing iOS 15 flutter/flutter#91633 and flutter/plugins#4429. We should add that version to the list of "Supported Google-tested platforms".

Expected fix

List iOS 13-15 as "Supported Google-tested platforms" and iOS 9-12 as "Best effort platforms tested by the community"

Additional context

No response