Drop support for Qt5/KF5

(also includes the commit of !13 (merged) as gitlab seems not offer a way to have relative MRs)

The plan for the next major KDE Gear release (the first "Qt6" one) for all the KDE games covered by it is to go full "in", i.e. only support Qt6/KF6 and never look back, thus also reducing complexity. Especially with the libraries libkdegames & libkmahjongg in mind.

A first Alpha release is planned for November. The final release as exception not 23.12, but delayed a bit, to be in sync with KF 6.0 & Plasma 6.0 in Feb 2024.

To prepare for that. all master branches would now switch to be Qt6-only already, to catch any issues in time as well as allow to also optimize for Qt6, without extra hooks to stay Qt5-compatible.

See also https://mail.kde.org/pipermail/kde-games-devel/2023-September/015462.html

The disadvantage is that until first rleases have happened, the KDE-made dependencies like here ECM, KI18n, KDocTools, LibKMahjongg one also has to build them from master branch as well, cannot use packages. Would that work for you? Given you just use the CMake macros and graphic resources here, perhaps you can also hack the buildsystem locally to keep using the KF5-variant for now while you do some more development?