Raising required Qt version to 5.15.2

[Christoph Cullmann]

Hi,

at the moment we require Qt 5.15.0 in Frameworks.

.2 introduced some new API functions for e.g. QStringView and
some merge requests are blocked due to that:

https://invent.kde.org/pim/kpimtextedit/-/merge_requests/11

Would it make sense to raise the bar to 5.15.2 for Frameworks?

Beside this, Qt 5.15.0 + .1 have a that broken QJSEngine that
even the trivial KTextEditor JS unit tests random fail
with GC crashs, I think it would make sense to really require
at least some "working" version.

Greetings
Christoph

-- 
Ignorance is bliss...
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