Update the cxx standard variant in Axom's spack package

[kennyweiss]

Our spack package has a cpp14 variant:
https://github.com/LLNL/axom/blob/df7fef005ffb2c40284ef22d0be789304ab51935/scripts/spack/packages/axom/package.py#L95

Since Axom now requires c++17 and will soon move to c++20, we should update this variant.
I think the convention is to use a cxxstd=<year> variant.

If we move to that option, do we need to keep the old cpp14 variant and update the when logic, or can we just remove it.

[white238]

I would mark all the versions deprecated that use that variant, wait for a spack release then remove it.