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

[Jaroslaw Kobus]

Hi All,

I propose that the Qt Task Tree module, released as a Technology Preview in Qt 6.11,
be promoted to "Maintained" status in Qt 6.12 according to:
https://contribute.qt-project.org/quips/14#becoming-a-maintained-module

The public API has been reviewed and full documentation has been made available.

Please share your feedback.

Thanks and best regards

Jarek