Demos should use `qt6_add_qml_module` instead of `qt6_add_resources`

Following QTBUG-87643, there is Gerrit commit 391143 about removing `set(CMAKE_AUTORCC ON)` from the demos and examples.

I noticed these examples use `qt6_add_resources`. Shouldn't they use `qt6_add_qml_module` instead? Given how the latter was advertised in the blog...