This PR adds a deprecation message when Android build is using the legacy "apply script method" way of applying Flutter's Gradle plugins (that is: [`flutter.gradle`](https://github.com/flutter/flutter/blob/3.16.0/packages/flutter_tools/gradle/flutter.gradle) and [`app_plugin_loader.gradle`](https://github.com/flutter/flutter/blob/3.16.0/packages/flutter_tools/gradle/app_plugin_loader.gradle)).

See also:
- #121541
  - in particular https://github.com/flutter/flutter/issues/121541#issuecomment-1836947311
- #135392
  - and PR that add the migration guide: [#9857](https://github.com/flutter/website/pull/9857)

- I think either `logger.error` or `logger.quiet` must be used, because all other error levels are not shown during `flutter build apk` (and that's what most people use).