[extension_google_sign_in_as_googleapis_auth] Update to google_sign_in 7

[stuartmorgan-g]

Updates the extension to work with the newly released google_sign_in 7.x, which reworked the API surface.

The extension method can no longer do as many of the steps, so its utility is lower. Longer term this may not be worth maintaining as a package, but for now this will simplify migration to google_sign_in 7.x for existing clients of this package.

Fixes https://github.com/flutter/flutter/issues/171048

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