Support for EL9/Alma9 linux, MG5_aMC@NLO v2.9.16

This release updates MG5_aMC@NLO to v2.9.16 and Rivet to v3.1.9.

To ensure a consistent software environment, we recommend using EL9/Alma9 linux and either an LCG or CMSSW release. Details are given in the README.