Minor CMake cleanup & modernization

Minor CMake cleanup & modernization

- Our minimum CMake version now supports BYPRODUCTS for
  add_custom_target(), which allows simplification of
  gmxCustomCommandUtilities.cmake.
- Specify USES_TERMINAL for targets that provide status output for nicer
  usability with Ninja.
- Rely on add_custom_command() automatically marking its outputs as
  generated.
- Update the minimum required version for building the template to match
  the main project.
- Do not unnecessarily put regressiontests MD5 to the cache.
- Fix the branch used for regression tests download for git builds.

Change-Id: Iee9a7e67d694724ad79cce9801bea670ef2f94c8