Lower CMake requirements

[richardmembarth]

CMake Error at samples/CMakeLists.txt:1 (cmake_minimum_required):
  CMake 3.14 or higher is required.  You are running version 3.xx

This is a very recent version found on no system. We should switch to a version that is available on most systems. Hipacc itself requires only 3.4. It seems that the requirement for 3.14 is
get_filename_component(_SRC_NAME_WLE ${_SRC} NAME_WLE)
NAME_WE is almost identical and requires only 3.4.
What else requires such a recent version?