Cloud: migrate from Google Sign-In to Credential Manager

[UweTrottmann]

https://developer.android.com/identity/sign-in/legacy-gsi-migration

But used through the FirebaseUI Auth library. So need to wait for it to update. Though it hasn't been since 2022.

Update 2025-03-19: release 9.0 which resolves this issues is now available https://github.com/firebase/FirebaseUI-Android/blob/master/docs/upgrade-to-9.0.md

Also looks like this will require at least SDK 23 (Android 6.0)