Bump policy_max CMake version to 3.31 to fix compatibility with CMake 4

[traversaro]

The soon to be released CMake 4.0 will raise an error for any project with cmake_minimum_required(VERSION 3.5), as it removed support for having exactly the same behaviour of CMake 3.5 (see https://cmake.org/cmake/help/latest/release/4.0.html#deprecated-and-removed-features).

To fix this, we add a policy_max argument to cmake_minimum_required, so that the project will remain compatible with CMake 3.5, but we will use the CMake policy of up to CMake 3.31 (this is kind of equivalent of setting all policy introduced up to CMake 3.31 to NEW.