[kio-extras] Port some QRegExp usage to QRegularExpression
Test Plan: Showing man pages in konqueror still works
Reviewers: dfaure, sitter, apol, meven
Reviewed By: meven
Subscribers: meven, kfm-devel, kde-frameworks-devel
Tags: Dolphin, Frameworks
Differential Revision: https://phabricator.kde.org/D28161