Qt version needed by Plasma 5.17

[Kai Uwe Broulik]

Hi,

I would prefer to keep Qt 5.12 supported, but maybe explicitly the 
latest patch release, i.e. 5.12.4 or so. Quickly skimming through the 
changelogs of Qt 5.13 I don't see any groundbreaking new features that I 
would want to use.

The Frameworks Qt requirement policy [1] was also updated recently to 
accomodate distributions better since there are Qt LTS releases now.

Cheers
Kai Uwe

[1] https://community.kde.org/Frameworks/Policies#Frameworks_Qt_requirements