Deprecate Kokkos_ENABLE_CUDA_UVM

[JBludau]

As we have the new alias SharedSpace we can deprecate Kokkos_ENABLE_CUDA_UVM.

[dalg24]

I am on board with the deprecation of Kokkos_ENABLE_CUDA_UVM in 4.0
We will need to elaborate on the why and what we want people to do instead in the documentation (we really need these deprecated code {2,3,4} pages)

I would have done this in two separate PRs as these are pretty much unrelated.
The deprecating/removing CudaUVMSpace::available() is uncontroversial as the function has become pointless (always returning true, not usable in constant expression, not provided by any other memory space, etc.)

[ldh4]

I also think these should be two separate PRs. One has to do with a cmake option while the other has to do with an obsolete function that just happens to be distantly relevant.

[JBludau]

Split into two prs. Still working on the documentation in kokkos/kokkos-core-wiki#201

[JBludau]

Please take a look at the documentation before merging
kokkos/kokkos-core-wiki#201

[JBludau]

ci seems unrelated (openmp.repeated_team_reduce)

[dalg24]

Need to coordinate with #5623
Same question here do we mean to define the option regardless of whether deprecated code 4 is enabled or not

(@JBludau I am not actually requesting any change at this time. I am just writing things down for bookkeeping. Maybe that is something we can harmonize as a follow up)

[JBludau]

@dalg24 sure, let's harmonize it as soon as we have a verdict