[copyjob] Port away from QLinkedList
Summary: QLinkedList will likely go away in Qt 6
Test Plan: Copied files from ftp server
Reviewers: Frameworks, dfaure
Reviewed By: dfaure
Subscribers: kde-frameworks-devel
Tags: Frameworks
Maniphest Tasks: T12087
Differential Revision: https://phabricator.kde.org/D25445