Replace flutter_dotenv to --dart-define-from-file

Dart define from file is the recommended way to pass environment variables to Dart and native code.
Can show a example on [this architecture](https://github.com/worldline-spain/flutter_architecture_template?tab=readme-ov-file#run-and-build) and a article [here](https://itnext.io/flutter-3-7-and-a-new-way-of-defining-compile-time-variables-f63db8a4f6e2)