Port QRegExp to QRegularExpression

Following the guidelines on https://doc.qt.io/qt-6/qregexp.html#porting-to-qregularexpression Mostly pretty straightforward changes, but I'm definitely not an expert here. There are changes all around, some of the ported codepaths have relevant tests, and they're ok (but the coverage's not too extensive). I've tried/will try to test everything works in day-to-day use, but hitting the relevant codepaths is often not that easy. Probably this should be merged a month before 3.0.2 so the changes will get some testing from nightly users, too.

Additionally, remove some unused code.