Transitioning CI builds of all non-Frameworks from Qt 5.9

[Ben Cooksley]

Hi all,

I've been informed by the PIM developers that they would like to bump
the dependency of the Qt version they use, from Qt 5.9 which it's on
currently, to Qt 5.10.

As a consequence, due to many KDE projects using PIM libraries, their
dependency on Qt will also be effectively bumped. To minimize the
maintenance cost of this bump for the CI system, I would like to bump
everyone up from Qt 5.9 to a newer version of Qt (otherwise we'll end
up chasing down build failures for a long time)

As most distributions have moved on from 5.10, and use Qt 5.11 now
(and will in many cases soon move to Qt 5.12), i'd like to suggest
that instead of going to Qt 5.10, we move straight to 5.11.

Because Frameworks has a two versions prior support policy, it'll
remain on 5.9 for now until it's ready to move up to 5.10 (assuming
5.12 is a usable Qt version)

If nobody has any objections, i'll proceed with this change in around
3 days time.

Cheers,
Ben