CMake 3.12 requirement

[pdimov]

CMake 3.12 is required here

histogram/CMakeLists.txt

Line 6 in 3afe76f

cmake_minimum_required(VERSION 3.12...3.27)

but I'm not sure why; from a quick look at the CML files nothing seems to need it.

Ubuntu 18.04 has CMake 3.10, and we're adding CI jobs to test with earlier CMake versions, so this came up.