Add warning for exec plugins using v1alpha1 (#1758)

Author: jrhouston

go.mod

@@ -68,7 +68,7 @@ require (
 	github.com/hashicorp/errwrap v1.1.0 // indirect
 	github.com/hashicorp/go-checkpoint v0.5.0 // indirect
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
-	github.com/hashicorp/go-cty v1.4.1-0.20200414143053-d3edf31b6320 // indirect
+	github.com/hashicorp/go-cty v1.4.1-0.20200414143053-d3edf31b6320
 	github.com/hashicorp/go-multierror v1.1.1 // indirect
 	github.com/hashicorp/go-uuid v1.0.2 // indirect
 	github.com/hashicorp/logutils v1.0.0 // indirect

kubernetes/provider.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"strconv"
 
+	"github.com/hashicorp/go-cty/cty"
 	"github.com/mitchellh/go-homedir"
 
 	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
@@ -127,6 +128,17 @@ func Provider() *schema.Provider {
 						"api_version": {
 							Type:     schema.TypeString,
 							Required: true,
+							ValidateDiagFunc: func(val interface{}, key cty.Path) diag.Diagnostics {
+								apiVersion := val.(string)
+								if apiVersion == "client.authentication.k8s.io/v1alpha1" {
+									return diag.Diagnostics{{
+										Severity: diag.Warning,
+										Summary:  "v1alpha1 of the client authentication API is deprecated, use v1beta1 or above",
+										Detail:   "v1alpha1 of the client authentication API will be removed in Kubernetes client versions 1.24 and above. You may need to update your exec plugin to use the latest version.",
+									}}
+								}
+								return nil
+							},
 						},
 						"command": {
 							Type:     schema.TypeString,