Not compatible with openmpi 5.0

OpenMPI has removed [ORTE and replaced it with PRTE in 5.0](https://docs.open-mpi.org/en/v5.0.0/news/news-v5.0.x.html). The old environment variables used such as `OMPI_MCA_orte_default_hostfile` need to be migrated to their `prte` equivalents such as `PRTE_MCA_prte_default_hostfile` if that is to be used from now on.

Start by locating where OMPI_MCA_orte_default_hostfile and related ORTE environment variables are handled. Compare those settings with the PRTE equivalents required by OpenMPI 5.0, then verify that the operator's MPI configuration works with the newer release. Done means OpenMPI 5.0 no longer encounters the removed ORTE variable names.

Written by the indexing model from the issue text.