[SpellChecking KCM] Fix the build

Summary:
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.

Test Plan: make && ctest

Reviewers: Plasma, bport, meven, crossi, ervin, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, bcooksley, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28232