MNT: Use range of allowed CMake versions

• Modern use of cmake_minimum_required allows for a range of minimum CMake version policies to be used, which allows for newer versions of CMake to be supported without errors as well as for more efficient builds with newer releases. Without the <policy_max> argument the version given for cmake_minimum_required will be enforced as the exact CMake policy versions used.
  • The optional <policy_max> argument was added in CMake v3.12 and is ignored in older releases, which allows it to be used even if an older CMake release / policy is used.
  • Remove use of FATAL_ERROR as it has been ignored since CMake v2.6.
  • c.f. https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html

Related, but not exactly motivating:

• https://github.com/conda-forge/hepmc3-feedstock/issues/26
• https://github.com/conda-forge/hepmc3-feedstock/pull/27