Use new FCM APIs

As of now FCM notifications are sent to https://fcm.googleapis.com/fcm/send which is a legacy API.

Messages should be sent to current API endpoint:

https://fcm.googleapis.com/v1/{parent=projects/*}/messages:send

[shamilbuzz]

You can change the API Endpoint for FCM by setting an attribute in push notification config as follows,

PUSH_NOTIFICATIONS_SETTINGS = {
    "FCM_API_KEY": "********",
    "FCM_POST_URL": "https://fcm.googleapis.com/v1/projects/{project_id}/messages:send",
     .......
     .......
}

[sevdog]

What concern me is that since APIs are different they accept different parameter format.

Looking into Firebase docs seems that API v1 has a different param signature from legacy one.

There is an official guide which explains why just changing URL is not enough.