Description
In #33968 preliminary support for Firebase notifications was added. However, that HTTP API is currently marked as legacy by Google (https://firebase.google.com/docs/cloud-messaging/http-server-ref) and it's encouraged to migrate to the new API (https://firebase.google.com/docs/cloud-messaging/migrate-v1). Maybe it's now the time to do it before 5.1 is released? The component is still marked as experimental, so I guess BC-breaks are acceptable?

Basically the new API is much more convenient when sending a notification to multiple device platforms (iOS, Android) at once. On the old one, you needed to target a specific device platform.