This location to change default could be the following in the flutter installation directory:

packages/flutter_tools/gradle/src/main/kotlin/FlutterExtension.kt: * Sets the ndkVersion used by default in Flutter app projects.

packages/flutter_tools/gradle/src/main/kotlin/FlutterExtension.kt: val ndkVersion: String = "26.3.11579264"

But I agree that there should be a better way to set this