[REG 6.7 -> 6.8] QSet::unite() no longer consistently picks equivalent elements from `other`

https://doc.qt.io/qt-6/qset.html#unite says:

Each item in the other set that isn't already in this set is inserted into this set.

This is not what the implementation does:

    for (const auto &e : std::as_const(other))
        insert(e);

(Created QTBUG-132503 to deal with this).

Since https://codereview.qt-project.org/c/qt/qtbase/+/531872, which set is *this and which is other is now "random", a regression from 6.7.

By Hyrum's Law, users may have some to expect that unite() works as implemented, meaning that equivalent elements of other overwrite elements of *this. Restore the old behavior.