Dolphin switch to KF6.

Merge branch 'kf6' into work/meven/dolphin-qt6-alpha

To be ready for KDE Gears 6 alpha

cc @felixernst

Following this we can update, sysadmin configuration and clean lefovers #ifdef

All MR tracking master will need to get rebased once merged, so to avoid unnecessary conflicts :

1. let's stop accepting MR for master (unless they are KF5 specific) accept only for kf6,
2. try to get merged MRs close to the finish line

Once there aren't many MR concerned by 2., then we can merge the MR.