x11/libxkbcommon: swtch to legacy rules on 11

x11/libxkbcommon was switched to use evdev rules in r528813, but FreeBSD 11
still needs the legacy rules. Switch back to the legacy rules, but only on
FreeBSD 11.
Adjust the UPDATING entry to indicate that the switch to evdev rules by
default is only on FreeBSD 12 and later, and that FreeBSD 11 still uses the
legacy rules.