qt6: Do not use deprecated QLibraryInfo::location in Qt 6
See https://doc.qt.io/qt-6/qtcore-changes-qt6.html#other-classes
https://github.com/bitcoin-core/gui/pull/623/commits/ad73447dc2bd04120c4eb7f5757613ad35a35cf5
qt6: Do not use deprecated high DPI attributes in Qt 6
In Qt 6, high DPI pixmaps and scaling are always enabled.
For the commit that cannot be backported yet, add a comment about missing backport (core-gui#556)
This is a backport of core-gui#623