[Development] Promoting Qt task tree module to "Maintained" state for Qt 6.12

[Jaroslaw Kobus]

> What kind of feedback have we received since we released the TP in Qt 6.11?
>
> Looking at JIRA, I see very few tickets for that component; https://qt-project.atlassian.net/browse/QTBUG-145110 might be hard to do if we are now locking down the API.
>

This ticket got already resolved: https://codereview.qt-project.org/c/qt/qttasktree/+/735940

> Also, what’s Qt Creator’s plan to use the upstream Qt TaskTree module? I haven’t followed closely, but the only relevant change in the past months seems to be
>
> https://codereview.qt-project.org/c/qt-creator/qt-creator/+/709964
>
> which still maintains the local copy. Qt Creator using the Qt version would be a strong argument for moving it out of TP, IMHO. While at the same time Creator not using it might be a reason to leave it in TP.
>

In meantime we have switched to the QtTaskTree taken from Qt (6.11), and the upcoming Creator 20.0 will use the module from Qt: https://codereview.qt-project.org/c/qt-creator/qt-creator/+/735780
For now we keep internally the copy of the task tree to support older builds with Qt 6.8 - 6.10.

>
> Volker