Add QRegion::rects() returning QSpan<const QRect>

QRegion used to have a QVector<QRect> rects(), which was removed for Qt 6.0, because since Qt 5.4 or so QRegion itself is iterable, so it's kinda useless. But this gives us the opportunity to use the vacant name rects() to return a QSpan<const QRect> instead. This has two benefits: a) Qt 5 code needs to be changed in fewer places and b) region.rects() is slightly more readable than iterating over the region itself.