upgrade-agent: set explicit k8s version

`kubeadm upgrade plan` takes an optional positional argument for the
target Kubernetes version [1], which is documented as

> To skip the internet check, pass in the optional [version] parameter

In light of RFC 015, we should not do an "internet check" for latest
versions, but stick to the version we're actually targeting with the
upgrade. Thus, we explicitly pass the version as an argument.

[1]: https://kubernetes.io/docs/reference/setup-tools/kubeadm/kubeadm-upgrade/#cmd-upgrade-plan

upgrade-agent/internal/server/server.go

@@ -110,9 +110,9 @@ func (s *Server) ExecuteUpdate(ctx context.Context, updateRequest *upgradeproto.
 		return nil, status.Errorf(codes.Internal, "unable to install the kubeadm binary: %s", err)
 	}
 
-	upgradeCmd := exec.CommandContext(ctx, "kubeadm", "upgrade", "plan")
+	upgradeCmd := exec.CommandContext(ctx, "kubeadm", "upgrade", "plan", updateRequest.WantedKubernetesVersion)
 	if out, err := upgradeCmd.CombinedOutput(); err != nil {
-		return nil, status.Errorf(codes.Internal, "unable to execute kubeadm upgrade plan: %s: %s", err, string(out))
+		return nil, status.Errorf(codes.Internal, "unable to execute kubeadm upgrade plan %s: %s: %s", updateRequest.WantedKubernetesVersion, err, string(out))
 	}
 
 	applyCmd := exec.CommandContext(ctx, "kubeadm", "upgrade", "apply", "--yes", updateRequest.WantedKubernetesVersion)