TaskTree: Make Group final

It's not meant to be derived from.

Change-Id: I356b3af743bc672f7a62e9a5d8516beadb85a68d
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/solutions/tasking/tasktree.h

@@ -200,7 +200,7 @@ private:
     TaskHandler m_taskHandler;
 };
 
-class TASKING_EXPORT Group : public GroupItem
+class TASKING_EXPORT Group final : public GroupItem
 {
 public:
     Group(const QList<GroupItem> &children) { addChildren(children); }