App type "gcm" in the config is confusing as we exclusively use FCM

[anoadragon453]

We switched a long while ago from Google Cloud Messaging (GCM) to Firebase Cloud Messaging (FCM). GCM essentially became FCM.

Thus APNs, FCM and WebPush are the only push backends we currently support in Sygnal. However, it can be confusing that people running the bridge need to specify gcm instead of fcm, as it's not clear which one is actually being used.

It may be nice to allow both gcm and fcm as a configurable app type, and to document that both mean "use fcm".

At the same time we can deprecate the use of gcm as a type in the config, and eventually drop it.