opt in to cmake policies through 3.31 to silence cmake warning

[spoonincode]

appbase requires c++17 which was first supported in cmake 3.8 so bump minimum version supported to 3.8. Also opt in for cmake policies through 3.31 to silence a warning that is showing up in latest cmake

CMake Deprecation Warning at libraries/appbase/CMakeLists.txt:2 (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.

(alternatively could possibly just opt in to CMP0167 instead of all the way through 3.31 but this seems fine..)