Use GmxManageMPI variables for gmxapi.

Use GmxManageMPI variables for gmxapi.

For minimal disruption to the release-2021 infrastructure,
let the `gmxapi` CMake target (libgmxapi) use
`${MPI_COMPILE_FLAGS}` and `${MPI_C_LIBRARIES}`
instead of the `MPI::MPI_*` target(s).

Note: This will conflict with the resolution to #3672.
Conflicts arising in a merge from `release-2021` to `master`
should defer to `master`.

Fixes #3896