[KDocTools] Port QRegExp to QRegularExpression

Summary:
xslt.cpp didn't really use QRegExp as the line using it has been commented
out since 2013, removed now.

Reviewers: Documentation, ltoscano, apol

Reviewed By: apol

Subscribers: apol, kde-doc-english, kde-frameworks-devel

Tags: Frameworks, Documentation

Maniphest Tasks: T12279

Differential Revision: https://phabricator.kde.org/D26156