As I understand it, C4D support for ProRender was implemented by Maxon and they have discontinued use. There is no source code for a plugin on the github page AFAIK so building for a later version of C4D isn't an option.
The good news is that you can use the latest ProRender version as a hydra delegate in C4D using USD.
I don't know much about how well that works as I use the traditional plugin on Maya. I had used USD with hydra-delegates in the early days of USD support in maya but I didn't like it, however 3D applications have much better support for USD now.
