archwiki: Switch captcha to time-based method

Up until now the captcha has depended on the exact output of the pacman
version command which could lead to multiple problematic scenarios:

    a) User uses testing repos (user pacman newer)
    b) Server is not instantly updated (user pacman newer)
    c) User system is not updated (user pacman older)

Circumvent this problem by switching to a time based captcha instead.

Signed-off-by: Christian Heusel <christian@heusel.eu>