Bump min deps of Qt & KF to 5.15.2 & 5.91
TO BE APPLIED AFTER 22.12 BRANCHING
@mwolff Picking up your initiative to get closer to Qt6/KF6, as proposed in https://mail.kde.org/pipermail/kdevelop-devel/2022-August/063726.html
Not going to drive myself, just doing some bits of work when I have time.