[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