Remove unneeded "cmake_minimum_required" in tools dir

[jphickey]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I reviewed the cFS README.md file to see if the feature is in the major future work.
• I performed a cursory search to see if the feature request is relevant, not redundant, nor in conflict with other tickets.

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/tools/CMakeLists.txt:2 (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
Remove this line here, it is not needed

Additional context
There is no need to specify a minimum version for this, it only adds the subdirectories which each have their own minimum (if specified). Otherwise it inherits the parent build minimum, which is fine.

Requester Info
Joseph Hickey, Vantage Systems, Inc.