docs(faq): update supported platforms

[amrgetment]

Description

Flutter currently supports Android API level 21+ and iOS 16.0+
https://docs.shorebird.dev/faq/#what-os-versions-does-shorebird-support

Requirements
change it to Flutter supports iOS 12+, and it will be 13+ soon not 16+

https://docs.flutter.dev/reference/supported-platforms

[eseidel]

Thanks!