Fix uses-sdk:minSdkVersion cannot be smaller than version 19

This error can be fixed by modifying build.gradle file in the flutter

In android studio: go to android>app>build.gradle and modify it as follows:

defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.example.untitled"
        // You can update the following values to match your application needs.
        // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration.
        minSdkVersion 21
        targetSdkVersion 31
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }