Bump org.jetbrains.kotlin:kotlin-gradle-plugin from 2.1.20 to 2.1.21 in /android/flutter_plugins/qrscanner_zxing/android

Bump org.jetbrains.kotlin:kotlin-gradle-plugin from 2.1.20 to 2.1.21 in /android/flutter_plugins/qrscanner_zxing/android #4712