messaging/registration-token-not-registered code example

Example: messaging/registration-token-not-registered

- The client app unregistered itself from FCM.
- The client app was automatically unregistered. This can happen if the user uninstalls the application or, on iOS, if the APNS Feedback Service reported the APNS token as invalid.
- The registration token expired. For example, Google might decide to refresh registration tokens or the APNS token may have expired for iOS devices.
- The client app was updated, but the new version is not configured to receive messages.