Deprecate the QStyleOptionProgressBar::orientation member

Its contents are already inside the "state" member.
Fixes a "TODO" for Qt 5 (bumping it to Qt 6, where we will be
able to break API).

[ChangeLog][QtWidgets][Styles] The usage of the
QStyleOptionProgressBar::orientation member has been deprecated.

Task-number: QTBUG-25121
Change-Id: Ie531bb2f2733b15a213e83c9706468e2bf829359
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>