ci: fix CI pipeline failures

[thatfiredev]

This PR should:

1. Update GH Actions to use Node 20 - the Firebase CLI no longer supports Node 16.
2. Update GH Actions to use Ubuntu instead of MacOS.
   The latest MacOS runner in GH Actions uses arm64 arch instead of x86_64, thus the android-emulator-runner action can't start an Android emulator.
   Ubuntu is said to be faster according to the android-emulator-runner action README and they also recommend enabling KVM.
3. Change the Android applicationId to com.example.firebase_snippets_app, which is the name expected by CI to clean up once the tests have finished running.
4. Disable animations on the Android device to make tests run faster.
5. Remove the github_sign_in flutter package which is unmaintained for ~3 years and not really being used in the project.