Update DNS-module for Hetzner to use the hcloud-provider

Hetzner's legacy DNS console (dns.hetzner.com) and its old DNS API have
been permanently shut down. All DNS zone management has been migrated
entirely to the main Hetzner Cloud Console.

Since I hadn't made any changes in the last couple of months I was
simply not affected by that change until very recently.
So, this change drops the 3rd-party provider I had used previously and
now applies the official `hcloud`-provider.

The migrations was a bit of a hassle, because I had to manually remove
the obsolete references from the Tofu-state. Then I checked the migrated
sources and decided to simply drop the migrated DNS-zones altogether and
simply run `tofu apply` once more.

variables.tf

@@ -20,10 +20,6 @@ variable "add_local_ip_to_ssh_allowed_ips" {
   description = "Whether to add the current local ip to the set of IPs which have access to the cluster via SSH."
 }
 
-variable "hetzner_dns_apitoken" {
-  type = string
-}
-
 variable "hetzner_cloud_apitoken" {
   type = string
 }