QXmlInputSource is deprecated in qt5.15. Port it to QXmlStreamReader
Summary: QXmlInputSource is deprecated in qt5.15
Test Plan: autotest ok
Reviewers: dfaure, apol
Reviewed By: apol
Subscribers: apol, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D26890