docs: Replace flutter packages get with flutter pub get

[tanyabouman]

Description
In a number of the tutorials, the command flutter packages get is used, rather than the now standard flutter pub get.

• https://bloclibrary.dev/tutorials/flutter-counter/
• https://bloclibrary.dev/tutorials/flutter-login/
• https://bloclibrary.dev/tutorials/flutter-weather/
• https://bloclibrary.dev/tutorials/flutter-timer/
• https://bloclibrary.dev/tutorials/flutter-firebase-login/
• https://bloclibrary.dev/tutorials/github-search/

It's also inconsistent with some of the tutorials that do use flutter pub get. https://bloclibrary.dev/tutorials/flutter-infinite-list/

I'm willing to make a PR for this issue.