add flatcar support for Hetzner (#9618)

contrib/terraform/hetzner/modules/kubernetes-cluster-flatcar/versions.tf

@@ -0,0 +1,13 @@
+terraform {
+  required_providers {
+    hcloud = {
+      source  = "hetznercloud/hcloud"
+    }
+    ct = {
+      source  = "poseidon/ct"
+    }
+    null = {
+      source  = "hashicorp/null"
+    }
+  }
+}