ECMAddQtDesignerPlugin: make it work with Qt6 too
Remove older Qt versions related code, looking at [1] it appears that distros that will be shipping ECM >= 5.94 aren't shipping Qt < 5.9. Thanks to Christophe Giboudeaux for pointing that out.
Tested with kwidgetsaddons, the plugin is built and installed as expected.
Edited by Ahmad Samir