Car rendering example breaks qtdoc builds with examples, if CMake version is below 3.21.1

The car rendering example requires CMake 3.21.1 as a minimum.
The requirement is stated in the CMakeLists.txt, and it is a functional requirement.
The example doesn't build with CMake 3.20.

Qt supports CMake 3.16 according to documentation.
OpenSuSE Leap 15.5 e.g. is provisioned with CMake 3.20.4.

The example should only be added, if the CMake requirement is met.