axom: Add cxxstd variant for C++20, python variant improvements

[bmhan12]

This PR:

• Adds cxxstd variant for C++20
• Marks cpp14 variant as deprecated, no longer a hard requirement
  • Relevant Discussion Comment: Update to C++20, spack 1.2.0 and package updates llnl/axom#1912 (comment)
• Improves python installation for python variant

Relates to:

• Update to C++20, spack 1.2.0 and package updates llnl/axom#1912 (C++20 changes)
• Adds axom module to python interface with axom.sidre llnl/axom#1896 (python changes)

[dtaller]

Thanks! I appreciate it!

[dtaller]

You may want to add conflicts if someone sets +cpp14 but also cxxstd=17 for example

[bmhan12]

The cpp14 variant in its current deprecated state is a misnomer.
It is a no-op the majority of the time, and it is only relevant in a narrow case where you want to build with C++14 with axom <= 0.6.1 & with blt <= 0.5.1 (with or without CUDA).
I've left the variant in but defaulted to false in order to not break some environments in the shared environments that have +cpp14 explicitly defined (configuration would break when spack determines variant no longer exists).