Identify and mark all deprecated/obsolete API that survived the transition from Qt 5 to Qt 6

Issue

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.

Task

Mark all such API with QT_DEPRECATED_SINCE() , Q_DECL_ENUMERATOR_DEPRECATED, or similar to ensure that these don't make it to Qt 7.

Extra notes

• Initial discussion: https://lists.qt-project.org/pipermail/development/2021-April/041176.html
• One systematic way to do this for class methods is to inspect all the classes listed at https://doc.qt.io/qt-6/obsoleteclasses.html – Compare each class' list of obsolete members against the Qt 5 version of the list.