Add troubleshooting note about potentially old "App signing key certificate" in use

[NorseGaud]

https://github.com/react-native-google-signin/google-signin-next/issues/145#issuecomment-3085870811

This was found by getting com.google.android.gms.common.api.ApiException: A non-recoverable sign in failure occurred from the Classic sign in, and then noSavedCredentialFound from the Universal Sign In.

[NorseGaud]

Looks like there is a good reason why some of my users could get through and not others. If the user has ever installed the app they still get the old signing key even if they completely uninstall and install again. https://stackoverflow.com/questions/72838558/google-play-signs-apk-with-legacy-signing-key-not-new-key seems to hint at this