[metalinkcreator] Properly set Save mode for save dialog with KIO 5.33+

KUrlRequester::fileDialog() is deprecated since 5.0.
Furthermore, setting the AcceptMode for the file dialog directly has no
effect any more since KIO 5.33.0, which introduced a new member function
to KUrlRequester exactly for this.

So, use the new KUrlRequester::setAcceptMode() instead if available.