Fix cmake_minimum_required to 3.16

[gullradriel]

In order to be aligned with all our other CMakeLists.txt I fixed cmake_minimum_required to 3.16.
It's killing the following warning when calling cmake:

  CMake Deprecation Warning at firmware/standalone/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

Note: all our other CMakeLists are using cmake_minimum_required to 3.16.