cmake: set policy version to 3.10

[pastalian]

This project does not rely on any behavior from CMake <3.10, and compatibility with CMake <3.10 is expected to be removed in future releases.

https://cmake.org/cmake/help/v4.0/manual/cmake-policies.7.html#policies-introduced-by-cmake-3-10

CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.