This PR temporarily pins the GoogleSignIn iOS SDK to version 7.0.0 in the darwin podspec configuration.
This'll enable our CI to keep going, until a more proper fix to `google_sign_in_ios` lands, with an update to the latest [7.1 version](https://github.com/google/GoogleSignIn-iOS/releases). See:
* https://github.com/flutter/flutter/issues/145866
For more information about the current CI failures.
## Testing
This PR should help with the tests that are currently broken in CI.