Update cmake mimimum required to something more recent (or remove)

[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/osal/src/unit-test-coverage/CMakeLists.txt:18 (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)

Additional context
In this case the OSAL main project currently lists 2.8.12 so it does not generate a warning here (unlike some other modules which specified 2.6.4) but the "unit-test-coverage" submodule is where the problem lies.

However, in this case, as this is just added as a subdirectory from the main build - it probably does not need a separate minimum_version at all. Suggestion would be to remove this line, it will inherit the minimum version from the top level project.

Requester Info
Joseph Hickey, Vantage Systems, Inc.

[skliper]

Note the base osal CMakeLists.txt won't trigger the warning yet, but it should probably be 3.5:

osal/CMakeLists.txt

Line 43 in 419c673

cmake_minimum_required(VERSION 2.8.12)