Update cmake mimimum required to something more recent

[jphickey]

Is your feature request related to a problem? Please describe.
The cmake_minimum_required command currently lists a very old version. In fact some of the CFE/CFS build scripts assume at least version 3.5 to support things like target-specific includes and flags.

Notably, when using a recent version of cmake (e.g. 3.20) this generates a warning that says:

CMake Deprecation Warning at /home/jphickey/code/cfecfs/dtn/apps/to_lab/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 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.

Describe the solution you'd like
Update the cmake_minimum_version to 3.5 (this would match CFE)

Describe alternatives you've considered
Disable warning?

Requester Info
Joseph Hickey, Vantage Systems, Inc.