Update OpenMPI custom logic for version 5

[bartoldeman]

The logic is a little simpler now, fortunately:

• Only use PMIx, no more PMI-1 or PMI-2
• Use UCX only on IB
• Use libfabric (ofi) only on OmniPath and avoid the OFI btl there just like in 4.1
• Set envvar to avoid warning if knem is not available for shared memory