Require CMake 3.5

[Tachi107]

As cmake-init requires policy CMP0063, it already effectively requires CMake 3.3. The oldest CMake version available in "mainstream" distributions is CMake 3.5, shipped with Ubuntu 16.04 (almost 7 years old, LTS support ended in 2021, in Extended security maintenance until 2026).

Unless there's a reason to require an older CMake version, I don't see why not to upgrade.

[scheibel]

Thanks for the pointer.
I created a follow-up issue to discuss the actual desired minimum version of CMake in #114.

Please note, that our use of CMP0063 will not require the use of CMake 3.3 as we implemented an additional policy existence check:

function(set_policy POL VAL)
    if(POLICY ${POL})
        cmake_policy(SET ${POL} ${VAL})
    endif()
endfunction(set_policy)

In the case of older CMake versions, where a policy is not yet defined, this will result in the OLD behavior instead of the parameter value, which should be manageable.