resolve two cmake warnings with latest cmake version

[spoonincode]

cmake 3.31 has deprecated rules for <3.10 resulting in a warning (even as part of spring),

CMake Deprecation Warning at 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.

cmake 3.12 is required for c++20 anyways, so bump minimum to 3.12.

I also noticed when building chainbase standalone (not as part of spring) that

CMake Warning (dev) at CMakeLists.txt:20 (FIND_PACKAGE):
  Policy CMP0167 is not set: The FindBoost module is removed.  Run "cmake
  --help-policy CMP0167" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

(this is actually shown in yellow as a full on warning). Go ahead and set CMP0167 to NEW to silence this warning: hopefully if you're using cmake 3.30+ you are using boost 1.70+. Just enabling this single policy felt lower risk than something like cmake_minimum_required( VERSION 3.12...3.30 ) which would have also made CMP0167 NEW but a lot of other policies NEW too.