Porting to qt6/kf6

OVERVIEW

Ported to Qt6/Kf6.

• Fixed CMakeLists.txt to use qt6/kf6
• For Qt6: Moved from Qt3DRender module to Qt3DCore because QBuffer, QAttribute, QGeometry classes got moved there
• For Kf6: Removed the interface instances from KTextEditor referenced in gcodeeditorwidget.h and .cpp because those were merged into the respective view and document class (https://api.kde.org/frameworks/ktexteditor/html/md__2builds_2websites_2api-kde-org_2sources_2ktexteditor_2docs_2porting__kf6.html)

BUG

Segmentation Fault a couple of seconds after successfully connecting to a marlin 3d printer