Migrate Hetzner webhook to the new Cloud API

[lentzi90]

Hetzner is shutting down the old DNS API. See vadimkim/cert-manager-webhook-hetzner#100.
It looks like the way forward is to this webhook instead: https://github.com/hetzner/cert-manager-webhook-hetzner/.
The old API shuts down in May. Migration has to happen before that.