Warn at configuration time if attempting to disable desul atomics and force using it

[dalg24]

Alternative to #5790 where we warn and just ignore the user setting instead of erroring out

Yields both

CMake Warning at cmake/kokkos_enable_options.cmake:167 (MESSAGE):
  Kokkos_ENABLE_IMPL_DESUL_ATOMICS option has been removed.  Desul atomics
  cannot be disabled.
Call Stack (most recent call first):
  cmake/kokkos_tribits.cmake:242 (INCLUDE)
  CMakeLists.txt:213 (KOKKOS_SETUP_BUILD_ENVIRONMENT)

<snip>

CMake Warning:
  Manually-specified variables were not used by the project:

    Kokkos_ENABLE_IMPL_DESUL_ATOMICS

EDIT Does not raise the Manually-specified variables were not used warning with the 2nd commit.
But I think this is good enough for now. In follow-up PRs will remove these variables and the warning will be back.

[crtrott]

I prefer warning slightly. Would have been ok with just removing the option and have CMake warn about unused variable.

[cz4rs]

No strong preference between the two approaches. Looks good from the technical side.

[nmm0]

👍

[dalg24]

Was step 1 in #5789