benchmarks/imb: upgrade to 2021.3

This port was very outdated: releases notes are available at
https://github.com/intel/mpi-benchmarks/releases.

The PDF is no more included but the user guide is available at
https://software.intel.com/en-us/imb-user-guide.

Also modernize the port and switch from OpenMPI 3 to OpenMPI4 (optiopnal,
MPICH still being the default).