Similar to the Matrix Push Gateway change, we should respond to Mastodon/etc. in a way that will make them delete the push subscription.

Mastodon:
https://github.com/mastodon/mastodon/blob/730bb3e211a84a2f30e3e2bbeae3f77149824a68/app/workers/web/push_notification_worker.rb#L35-L46

Common-Proxies:
https://github.com/UnifiedPush/common-proxies/blob/main/gateway/matrix.go#L63

/cc FYI @karmanyaahm @p1gp1g