terraform-providers.kubernetes-alpha: remove

This has graduated to beta, and the kubernetes_manifest resource has
moved into terraform-provider-kubernetes.

To use it, you must be enabled in the provider block:

```hcl
provider "kubernetes" {
  experiments {
    manifest_resource = true
  }
  config_path = "~/.kube/config"
}
```

pkgs/applications/networking/cluster/terraform-providers/default.nix

@@ -57,6 +57,7 @@ let
     cloudfoundry = callPackage ./cloudfoundry {};
     gandi = callPackage ./gandi {};
     hcloud = callPackage ./hcloud {};
+    kubernetes-alpha = throw "This has been merged as beta into the kubernetes provider. See https://www.hashicorp.com/blog/beta-support-for-crds-in-the-terraform-provider-for-kubernetes for details";
     libvirt = callPackage ./libvirt {};
     linuxbox = callPackage ./linuxbox {};
     lxd = callPackage ./lxd {};

pkgs/applications/networking/cluster/terraform-providers/providers.json

@@ -563,15 +563,6 @@
     "vendorSha256": null,
     "version": "2.4.1"
   },
-  "kubernetes-alpha": {
-    "owner": "hashicorp",
-    "provider-source-address": "registry.terraform.io/hashicorp/kubernetes-alpha",
-    "repo": "terraform-provider-kubernetes-alpha",
-    "rev": "v0.5.0",
-    "sha256": "0yqm3wlya69w9g9kzgvm28mbbwp6wik51syjnbnj8dis5kspx8gd",
-    "vendorSha256": null,
-    "version": "0.5.0"
-  },
   "launchdarkly": {
     "owner": "terraform-providers",
     "repo": "terraform-provider-launchdarkly",