Bump cmake_minimum_required version to 3.5

[jlblancoc]

With recent versions of cmake (>? 3.28), one gets this warning for cmake required version <3.5:

CMake Deprecation Warning at 3rdparty/robin-map/CMakeLists.txt:1 (cmake_minimum_required):
Compatibility with CMake < 3.5 will be removed from a future version of
CMake.

The oldest active Ubuntu distro (20.04) already shipped cmake 3.16.3, so I hope there is no issue in upgrading this required version to the minimum that cleans up the warning.

[Tessil]

Thanks, it looks reasonable to increase it