`get_validation_foo`: convert 'Final' to 'RC' for >23
    
    With Pungi 4 (Fedora 24+), the 'Final' milestone kinda went
    away and we got 'RC' instead. But people are probably quite
    used to using 'Final' in relval by now and might even have it
    scripted. So instead of leaving this as a bear trap, let's
    convert it, at least in the high-level `get_validation_page`
    and `get_validation_event`.