CMAKE_SYSTEM_VERSION is not correctly set when autodetected

[czoido]

As stated in the CMake documentation when in Apple systems CMAKE_SYSTEM_VERSION is the version for the Darwin version, but we are setting the sdk version, which does not seem correct. In general the best approach for cross-building would be to set these variables via the conf or user toolchain, but we may fix this by converting from the OS version to the Darwin version.

https://github.com/conan-io/conan/blob/151c1dd3b5c45b2c968a1bbe50b220027d415a37/conan/tools/cmake/toolchain/blocks.py#L905