docs: Update Google signInWithGoogle method to match 7.xx API

Author: being-eyram

docs/auth/federated-auth.md

@@ -37,16 +37,13 @@ Ensure the "Google" sign-in provider is enabled on the [Firebase Console](https:
 
   Future<UserCredential> signInWithGoogle() async {
     // Trigger the authentication flow
-    final GoogleSignInAccount? googleUser = await GoogleSignIn().signIn();
+    final GoogleSignInAccount? googleUser = await GoogleSignIn.instance.authenticate();
 
     // Obtain the auth details from the request
-    final GoogleSignInAuthentication? googleAuth = await googleUser?.authentication;
+    final GoogleSignInAuthentication googleAuth = googleUser.authentication;
 
     // Create a new credential
-    final credential = GoogleAuthProvider.credential(
-      accessToken: googleAuth?.accessToken,
-      idToken: googleAuth?.idToken,
-    );
+    final credential = GoogleAuthProvider.credential(idToken: googleAuth.idToken);
 
     // Once signed in, return the UserCredential
     return await FirebaseAuth.instance.signInWithCredential(credential);