Fix CMake warnings regarding compatibility with older versions

When running CMake, there are warnings for our external libraries which should be fixed:

CMake Deprecation Warning at external/fmt/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.