Drop Legacy FCM support.

[fujiwara]

https://firebase.google.com/docs/cloud-messaging/migrate-v1

Apps using the deprecated FCM legacy APIs for HTTP and XMPP should migrate to the HTTP v1 API at the earliest opportunity. Sending messages (including upstream messages) with those APIs was deprecated on June 20, 2023, and will be removed in June 2024.

FCM Legacy API will be removed soon. Gunfish will drop supports the legacy API (/push/fcm).