bump cmake minimum to 3.12 as needed for c++20

[spoonincode]

chainbase already requires C++20, so require the first version of cmake that supports C++20. Resolves a warning from latest version of cmake,

CMake Deprecation Warning at libraries/chainbase/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.

[spoonincode]

whoops duplicate of #55