Port to Qt/KF 6 and ICU 75

When trying to rebuild against ICU 75 I noticed that the Qt/KF5-based builds still use C++11. This breaks with ICU 75 which would require C++17. The Qt/KF6-based builds would select C++17 but don't compile on 0.8.0. Maybe it just is time to tag a new release (if Qt/KF6-based builds compile on master)?