docs(faq): update supported platforms

**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 <img width="929" alt="Image" src="https://github.com/user-attachments/assets/8b73f6e5-4f74-4a83-89da-95cc41772e9b" />