Balk at using Kubernetes versions too new to be supported

upup/pkg/fi/cloudup/apply_cluster.go

@@ -1053,6 +1053,29 @@ func (c *ApplyClusterCmd) validateKubernetesVersion() error {
 		return nil
 	}
 
+	kopsVersion, err := semver.Parse(kopsbase.KOPS_RELEASE_VERSION)
+	if err != nil {
+		klog.Warningf("unable to parse kops version %q", kopsVersion)
+	} else {
+		tooNewVersion := kopsVersion
+		tooNewVersion.Minor += 1
+		tooNewVersion.Pre = nil
+		tooNewVersion.Build = nil
+		if util.IsKubernetesGTE(tooNewVersion.String(), *parsed) {
+			fmt.Printf("\n")
+			fmt.Printf("%s\n", starline)
+			fmt.Printf("\n")
+			fmt.Printf("This version of kubernetes is not yet supported; upgrading kops is required\n")
+			fmt.Printf("(you can bypass this check by exporting KOPS_RUN_TOO_NEW_VERSION)\n")
+			fmt.Printf("\n")
+			fmt.Printf("%s\n", starline)
+			fmt.Printf("\n")
+		}
+		if os.Getenv("KOPS_RUN_TOO_NEW_VERSION") == "" {
+			return fmt.Errorf("kops upgrade is required")
+		}
+	}
+
 	if !util.IsKubernetesGTE(OldestSupportedKubernetesVersion, *parsed) {
 		fmt.Printf("This version of Kubernetes is no longer supported; upgrading Kubernetes is required\n")
 		fmt.Printf("\n")