CMake 3.27 deprecation message on build

[eratchia]

Hi,

Cmake version 3.27 and up is dropping compatibility with versions older than 3.5 and displaying a deprecation message.
From https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html#policy-settings:

Changed in version 3.27: Compatibility with versions of CMake older than 3.5 is deprecated. Calls to cmake_minimum_required(VERSION) or cmake_policy(VERSION) that do not specify at least 3.5 as their policy version (optionally via ...) will produce a deprecation warning in CMake 3.27 and above.

Building this project already requires at least CMake 3.5 except for cmake/AddSelfContainmentTest.cmake which requires only version 3.1 which results in a deprecation message while using CMake 3.27 and up.

Changing the requirement from 3.1 to 3.5 would improve the experience of using the project with CMake versions 3.27 and up while not impacting the range of CMake versions with which the project is usable.