Update AppMan licensing to GPL+Commercial

AppMan has a critical dependency to Qt Wayland that has both GPL+Commercial headers. Also, we will no longer have a dedicated Qt AS license headers but will use generic commercial license in Qt 6. --> Thus only feasible licensing for AppMan itself is GPL+Commercial headers https://code.qt.io/cgit/qt/qtqa.git/tree/tests/prebuild/license/templates/header.GPL

This change needs to be done by Qt 6.2.

This change was in fact already announced in 2019 https://www.qt.io/blog/change-in-open-source-licensing-of-qt-wayland-compositor-qt-application-manager-and-qt-pdf but it was not executed to the full.