Finish migration to Play Games Services v2.

[theLee3]

It turns out that Play Games Services v2 includes SavedGame access by default, so we can do away with separate sign-ins and move fully to v2. This wraps up the transition with the following:

• Fix isSignedIn getter to use the v2 isAuthenticated check.
• Remove use of GoogleSignIn and related classes.
• Replace silent/explicit sign in methods with a simple v2 signIn.
• Remove enableSavedGames argument from signIn method.
• Remove isAuthenticated check from signIn. (In v2, this should only be used to check sign-in status by the end dev. As sign-in happens automatically, signIn should only be a fallback sign-in option used by the end dev as needed, i.e. if a sign-in button is pressed.
• Remove signOut method, as there is no concept of a sign-out method in v2 from a developer perspective. This is now only handled at the system level.
• Bump version to 4.0.0. As the migration to v2 is a breaking change (but a great one!) we should bump up the major version of the package.

[theLee3]

I also think that the 4.0 release is a great time to remove the deprecated code from the iOS/macOS implementation, as well as force migration to the new class structure (Auth, Player, etc.) and drop support of the monolithic GamesServices class as there will be no need to support <3.0.0 usage once >=4.0.0.

[Abedalkareem]

Nice work! All looks good, we need just to do one test before merging this.
#119 (comment)

@theLee3 Sure, I'm thinking the same maybe we should start removing the legacy code 🚀.