examples/cancel: Fix to securely lookup subject
examples/cancel: Fix to securely lookup subject

This is just an uninstalled example, but it needed the same fix as was
done for pkexec with the 3b12cfac29dddd27f1f166a7574d8374cc1dccf2
commit.

This drops use of deprecated API.

https://bugs.freedesktop.org/show_bug.cgi?id=69538

magcius/polkit,magcius/polkit,cgwalters/polkit,krytarowski/polkit,cgwalters/polkit,cgwalters/polkit,krytarowski/polkit,krytarowski/polkit,cgwalters/polkit,magcius/polkit,cgwalters/polkit,krytarowski/polkit,magcius/polkit,magcius/polkit,krytarowski/polkit