[Qt-creator] Bumping minimum Qt version needed to build Qt Creator to Qt 5.14

[André Pönitz]

Short version:

We'd like to bump the minimum Qt version needed to build Qt Creator itself
to Qt 5.14 starting with the upcoming Qt Creator 4.13 (the current 'master'
branch in the repo)

This would not affect the versions of Qt that can be used for applications
build with Qt Creator. These would remain the same, i.e. in practice Qt 4.8
and any Qt 5.x.


Longer version:

During the transition from Qt 4 to 5  Qt Creator was compilable with both
major versions for quite a while (more than a year IIRC). This helped e.g.
to catch regressions in Qt, and also to not have to port the code base
itself in one shot. That was good.

We had a look on what it would take to do the same for the transition to
Qt 6, and while it looks like the ride would be a bit more bumpy this time
due to the amount of removed or changed API in Qt 6 this might still be
possible.

Since for some of the changes replacements were (re-)introduced recently,
it would simplify the setup if we could use these. Currently it looks like
we might get away with 5.14 but it *could* be that we'd end up with 5.15
as a requirement. 5.14 would incidentally also fit the normal scheme of
having the minimum set at "latest released Qt LTS minus one minor" that
has been in place during the last years.

Andre'