Dropping support for Android KitKat

Flutter's minimum supported Android version is now Lollipop (API 21). Beginning with Flutter's 3.22 stable release, Flutter will no longer work on devices running Android KitKat (API 19).

The context, purpose, and description of this deprecation can be found in the go/rfc-android-k-deprecation design document.

Flutter developers targeting Android will need to increase the minSdkVersion in their build.gradle and AndroidManifest.xml files from 19 to at least 21.

In stable release: 3.22