[cmake] use correct variable for Kokkos backend

What this MR does / why does DuMux need it:

CMake variables (for detected packages) were renamed to include DUMUX_ in !3666 (merged), but the usage of HAVE_KOKKOS in parallel_for.hh was missed.

Should be backported to release 3.8, where the original change was made.

Notes for the reviewer

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