FCM v1 HTTP API as default when fcm is enabled

[sprda]

zmeventnotification.pl is already coded to leverage the FCMv1 protocol via config 'use_fcmv1 = yes'. This should be the default protocol when fcm is enabled.
https://firebase.google.com/docs/cloud-messaging/send-message?authuser=0#send_using_the_fcm_v1_http_api

Code Base:
Provide depreciation warning (if needed), and code cleanup for FCM legacy HTTP API.
https://firebase.google.com/docs/cloud-messaging/send-message?authuser=0#send_using_the_fcm_legacy_http_api

[sprda]

Also addresses #418