Update CMake version to 2.8.12

[amaiorano]

With CMake 3.22.1, when generating the project, CMake now outputs the following:

C:\code\cppdap\build>cmake ..
CMake Deprecation Warning at CMakeLists.txt:16 (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.

The current min version is set to 2.8. Can we bump it to 2.8.12 to get rid of this warning?

[ben-clayton]

I believe the old CMake version stems from the kokoro images. We don't have a custom image for Mac, but it's possible that this has been since updated. Let's just try, and see what happens?

[amaiorano]

Sure, I'll make a PR soonish :)

[waywardmonkeys]

This was updated to a much more recent version of cmake (3.11) in 5b79645

I think this can be closed.

[ben-clayton]

Thanks. Closing.