Update GoogleSignIn dependency to 6.0

[aerych]

Now that GoogleSignIn has been updated with M1 support, let's update the version used by the authenticator pod to 6.0.x.
Alternatively, if there is adequate support via https://developer.apple.com/documentation/authenticationservices we could use that instead, but may need more investigation.

[jkmassel]

I'd personally prefer using the iOS native approach given the number of libraries that are pulled in as part of this 😅

There is some work-in-progress on this that myself and @ctarda looked at, if that's useful at all?

[aerych]

Tough to say how close the WIP got just by looking at the branch. Did y'all have a sense for how far a long it was? @ctarda, @jkmassel