Update native Google OAuth docs for Flutter

# Improve documentation ## Link https://supabase.com/docs/guides/auth/social-login/auth-google?queryGroups=platform&platform=flutter#signing-users-in ## Describe the problem The google_sign_in package used in the Flutter docs for Google OAuth has a new major version (7.x as of 6 days ago) with a lot of breaking changes, which make the currently available Supabase docs outdated. ## Describe the improvement I think I managed to create a decent update based on the migration I read for the package: https://github.com/flutter/packages/blob/main/packages/google_sign_in/google_sign_in/MIGRATION.md ## Additional context Scopes can be found via the following link: https://developers.google.com/identity/protocols/oauth2/scopes?hl=en#google-sign-in I chose `email` and `profile` because it made the most sense based on what I assume is the minimum Supabase would need to create a user. ## Code example This is my first time trying to contribute to docs, so any suggestions are welcome ```dart import 'package:google_sign_in/google_sign_in.dart'; import 'package:supabase_flutter/supabase_flutter.dart'; final supabaseClient = Supabase.instance.client; Future<void> nativeGoogleSignIn() async { /// TODO: update the Web client ID with your own. /// /// Web Client ID that you registered with Google Cloud. const webClientId = 'my-web.apps.googleusercontent.com'; /// TODO: update the iOS client ID with your own. /// /// iOS Client ID that you registered with Google Cloud. const iosClientId = 'my-ios.apps.googleusercontent.com'; final scopes = ['email', 'profile']; final googleSignIn = GoogleSignIn.instance; await googleSignIn.initialize( serverClientId: webClientId, clientId: iosClientId, ); final googleUser = await googleSignIn.attemptLightweightAuthentication(); // or await googleSignIn.authenticate(); which will return a GoogleSignInAccount or throw an exception if (googleUser == null) { throw AuthException('Failed to sign in with Google.'); }…