mdspan cmake ignores `CMAKE_CXX_STANDARD`

[nmm0]

Instead you need to provide MDSPAN_CXX_STANDARD. We should accept both.

[dalg24]

Why do we need MDSPAN_CXX_STANDARD in the first place?

[nmm0]

Oh I guess we don't. I was thinking about it breaking people's build scripts but we are header only so it won't matter.