FCM Does not work due to old firebase-admin version

[MichaelMarner]

FCM has deprecated their legacy HTTP APIs for sending messages:

https://firebase.google.com/docs/cloud-messaging/migrate-v1?hl=en&authuser=0

This has resulted in push notifications failing to send through this package.

Looking at Google's FAQ, we need to be using firebase-admin 11.7 or newer:

https://firebase.google.com/support/faq?authuser=0#deprecated-api-shutdown

Updating from 9.7 to 10 has breaking changes:

https://firebase.google.com/docs/admin/migrate-node-v10?authuser=0