fix: Push Notification sending API

[tuliomir]

During a bug investigation on the Mobile Wallet push notifications #589, it was identified that the sendMulticast(message) method being used to send the notifications was deprecated and no longer works.

The main recommendation is to use the sendEachForMulticast() method instead.

References

• Official docs for the Firebase Admin dependency ( link )
• PR where the deprecation happened: Deprecate sendAll and sendMulticast firebase/firebase-admin-node#2094
• Community issue with a problem similar to ours messaging/unknown-error when using sendEachForMulticast firebase/firebase-admin-node#2687
• Sample solution in Flutter code, similar to what will probably be done here too ( link )
• Related alert received through our monitoring service ( link )