Build QtBase with QT_DISABLE_DEPRECATED_BEFORE = current Qt version

Once we fix the majority of problems with deprecated methods from 6.0 (see QTBUG-104857), we need to try to raise the deprecation version until we reach the current Qt version (dev has 6.5 in time of creating this issue).

The scope of this task is to find and fix all the deprecation-related issues, or probably create subtasks, if some major issues are found (a good example is QTBUG-105048).

Note that it might not be possible to build QtBase with deprecated methods for some intermediate versions between 6.0 and current (6.5). For example, the above mentioned QTBUG-105048 requires deprecating some more APIs and providing replacements for them. So it can't be cherry-picked to older Qt versions.