build: set minimum required CMake to 3.5

[fanquake]

Avoids:

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

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

[miniupnp]

https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html

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.

Changed in version 3.19: Compatibility with versions of CMake older than 2.8.12 is deprecated. Calls to cmake_minimum_required(VERSION) or cmake_policy(VERSION) that do not specify at least 2.8.12 as their policy version (optionally via ...) will produce a deprecation warning in CMake 3.19 and above.

So I think we should use cmake_minimum_required(VERSION 3.5)

[fanquake]

So I think we should use cmake_minimum_required(VERSION 3.5)

Sure. Changed to 3.5.