Precise the gcc version required by CUDA toolkit

When compiling with the gcc version installed by default in conda, we get the error:

$HOME/miniforge3/include/crt/host_config.h:132:2: error: #error -- unsupported GNU version! gcc versions later than 11 are not supported! The nvcc flag '-allow-unsupported-compiler' can be used to override this version check; however, using an unsupported host compiler may cause compilation failure or incorrect run time execution. Use at your own risk.

So the Readme is updated to match the gcc version to the version required by cuda-toolkit.