fix(firebase_auth): use correct UTC time from server for `currentUser?.metadata.creationTime` & `currentUser?.metadata.lastSignInTime`

[russellwheatley]

Description

// On firebase-admin (server)
console.log(user.metadata.creationTime); // Mon, 18 Jul 2022 10:17:31 GMT

// As currently implemented
print('${DateTime.fromMillisecondsSinceEpoch(_creationTimestamp!)}'); // 2022-07-18 11:17:31.699

// Fix
print('${DateTime.fromMillisecondsSinceEpoch(_creationTimestamp!, isUtc: true)}'); // 2022-07-18 10:17:31.699Z

Related Issues

fixes https://github.com/firebase/flutterfire/issues/9162

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