Port dbjobsthread.cpp to not use deprecated const_iterator "+" operator

The operator https://doc.qt.io/qt-5/qset-const-iterator-obsolete.html#operator-2b is deprecated and we used it once here https://invent.kde.org/graphics/digikam/-/blob/master/core/libs/database/dbjobs/dbjobsthread.cpp#L248

I have changed implementation that avoids the situation to use this operator.

As suggested by @mqualmann, implementation uses ++ operator on the iterator.