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.

conan/conan/tools/cmake/toolchain/blocks.py

Line 905 in 151c1dd

_system_version = self._conanfile.settings.get_safe("os.sdk_version")