cmd/tailscale/cli,ipn: mention available update in "tailscale status" (#9205)

Cache the last `ClientVersion` value that was received from coordination
server and pass it in the localapi `/status` response.
When running `tailscale status`, print a message if `RunningAsLatest` is
`false`.

Updates #6907

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscale/cli/status.go

@@ -236,6 +236,9 @@ func runStatus(ctx context.Context, args []string) error {
 		printHealth()
 	}
 	printFunnelStatus(ctx)
+	if cv := st.ClientVersion; cv != nil && !cv.RunningLatest && cv.LatestVersion != "" {
+		printf("# New Tailscale version is available: %q, run `tailscale update` to update.\n", cv.LatestVersion)
+	}
 	return nil
 }