Drop support for intermediate standards in CMake

[jjwilke]

I don't think we need users to specify 1Y, 1Z, or 2A to get intermediate standards.
CMake is smart enough (in this case) to recognize when a compiler version has partial support and will "downgrade" 20 -> 2a, 17 -> 1z, and 14 -> 1y where appropriate.

This occurs, for example, with GCC 6,7 where you can request 17 but will get -std=c++1z.

After nvcc_wrapper fixes, this means we can ALWAYS set -std flags as compiler features without choosing flags. This finally fixes the supreme annoyance of multiple -std flags in downstream projects in all cases.

[jjwilke]

Related to issue #2317 and PR #2311

[jjwilke]

We will have to be careful with versions here. If we get a request for C++20, we need to make sure we have CMake >= 3.12.

[jjwilke]

The only allowed inputs now are 11, 14, 17, and 20. The user would never ask for 1Y, 1Z, or 2A. There is some internal logic if Kokkos is forced to pick the flag itself (doesn't get one from CMake) that still handles intermediate standards.