fix: drop duplicate required_providers from generated backend.tf

The module's versions.tf and the root.hcl-generated backend.tf both
declared required_providers, which OpenTofu rejects ("A module may have
only one required providers configuration"), so terragrunt init/plan
failed. Keep them in the module's versions.tf (authentik + vault) and
generate only the backend + provider blocks.

environments/root.hcl

@@ -25,16 +25,8 @@ terraform {
     ca_file = "/etc/pki/tls/certs/ca-bundle.crt"
   }
   required_version = ">= 1.10"
-  required_providers {
-    authentik = {
-      source  = "goauthentik/authentik"
-      version = "2026.5.0"
-    }
-    vault = {
-      source  = "hashicorp/vault"
-      version = ">= 4.0.0"
-    }
-  }
+  # required_providers are declared in the module's versions.tf; declaring them
+  # here as well is a duplicate configuration (OpenTofu permits only one).
 }
 EOF
 }