Updated pubspec. Replaced deprecated keys

[britov]

No description provided.

[davetoxa]

@dimmetrius подскажи пожалуйста, какая у вас версия flutter? этот PR не сломает проект?
хочется обновить потихонечку под flutter 2.0

[britov]

Без этого изменения dart pub publish не публикует пакет

Package validation found the following error:
* In pubspec.yaml the flutter.plugin.{androidPackage,iosPrefix,pluginClass} keys are deprecated. Instead use the flutter.plugin.platforms key introduced in Flutter 1.10.0
  
  See https://flutter.dev/docs/development/packages-and-plugins/developing-packages#plugin
Sorry, your package is missing a requirement and can't be published yet.
For more information, see: https://dart.dev/tools/pub/cmd/pub-lish.