cmd/tailscale/cli: [up] compact QR code

Reduce QR code from 35 to 15 lines by using half-block characters,
disabling border and switching to negative.

See #6096 for potentially of reducing it further (11 lines)

Signed-off-by: Oren Tirosh <orent@hishome.net>

cmd/tailscale/cli/up.go

@@ -636,10 +636,11 @@ func runUp(ctx context.Context, cmd string, args []string, upArgs upArgsT) (retE
 					fmt.Fprintf(Stderr, "\nTo authenticate, visit:\n\n\t%s\n\n", *url)
 					if upArgs.qr {
 						q, err := qrcode.New(*url, qrcode.Medium)
+						q.DisableBorder = true
 						if err != nil {
 							log.Printf("QR code error: %v", err)
 						} else {
-							fmt.Fprintf(Stderr, "%s\n", q.ToString(false))
+							fmt.Fprintf(Stderr, "%s\n", q.ToSmallString(true))
 						}
 					}
 				}