Replace Buffer's CPtr by std::span

Looking at `CPtr` in https://github.com/ComputationalRadiationPhysics/picongpu/blob/ef239f04a4d9458609ef50e72f84140ba2e6d4a7/include/pmacc/memory/buffers/Buffer.hpp#L197 it is the same as a `std::span` with a reduced interface. Should we consider moving to `std::span`? I've been using it on device in other codes just fine.