[Development] Deprecated/Obsolete API that survived the transition from Qt 5 to Qt 6

[Sze Howe Koh]

Some parts of the Qt API have been documented as obsolete for a long
time, but were not removed before Qt 6.0.0 was released.

For example, the following 2 pages contain exactly the same list:

* https://doc.qt.io/qt-5/qmessagebox-obsolete.html
* https://doc.qt.io/qt-6/qmessagebox-obsolete.html

QLocale::countriesForLanguage() is another example.

These probably slipped through the cracks because they were only
tagged "\obsolete" for QDoc but didn't get tagged with macros like
QT_DEPRECATED_SINCE() or QT_VERSION_CHECK().

Is it acceptable to remove them during Qt 6's lifetime? Or should we
wait till Qt 7?


Regards,
Sze-Howe