Upgrade min Flutter, Dart and Android SDK versions and upgrade dependencies

* Upgrade min Dart SDK from 2.15 to 2.17
* Upgrade minimum Flutter version from 2.8 to 3.0 – this was required as a result of updating the other dependencies
* Change in Android SDK versions:
   * Min SDK version from 16 to 21
   * Compile SDK version from 30 to 31
* Kotlin version from 1.4 to 1.7
* OkHttp from 4.9 to 4.10
* Upgrade flutter_lints from 1 to 2