Improve handling of cxxstd variants in RADIUSS packages

[adrienbernede]

Raja expresses constraints on variant values using conflicts:

spack-packages/repos/spack_repo/builtin/packages/raja/package.py

Line 260 in d0045f6

default="20",

Umpire expresses constraints on variant values with repeated definitions of the variant:

spack-packages/repos/spack_repo/builtin/packages/umpire/package.py

Line 238 in d0045f6

"cxxstd",

(Namely, detail the exact compatibility between versions and C++ standards without using conflicts).

Spack also provides "conditional" values to express constraints directly in the variant definition.
https://spack.readthedocs.io/en/latest/packaging_guide_creation.html#conditional-possible-values

The goal here is to determine whether on solution is better than the other and why.

Originally posted by @adrienbernede in #5716 (comment)

[adrienbernede]

If I could, I would assign it to myself :)