Change the minimum required Wayland version back to 1.2

As discussed, Wayland 1.3 is not common in distributions yet, and
that is a problem for the C.I. machines.

Change-Id: Iec5c6d8ae1d1a50199f66d45ca9269694db78efe
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>