Google announced that they are deprecating their legacy Google Firebase Cloud Messaging APIs starting?June 20 2023, culminating in a full removal of this functionality after June 20 2024. Google has released a new version of this FCM API labelled as Firebase Cloud Messaging API V1 (or HTTP v1).
https://firebase.google.com/docs/cloud-messaging/migrate-v1
The old API used by the plugin is no longer usable, there is no more server API key.
