Use KF5::AuthCore instead of KF5::Auth

Summary:
It's a smaller library and provides everything needed here.

Unfortunately introduced just in KF5.56 so i will understand if you tell me to ping you back in something like 6 months :)

Reviewers: areinholdt

Reviewed By: areinholdt

Subscribers: areinholdt, Smb4K

Differential Revision: https://phabricator.kde.org/D19071