QList/QSet iterator-based ctors are available since Qt 5.14, so we
could make the code conditional based on that to keep it building
against older Qt versions. However I think the reason behind converting
from QStringList to QSet was to remove duplicates, so use
QStringList::removeDuplicates() and sort() instead.
Bump KF5 min. required version to 5.69.0 because of Sonnet/ConfigView
which first appeared in that version.