Add simpler jet fold hash

This should change no existing outputs and is to support future versions of the flavor tagging GNN. I'm adding it to 24.0 because we're trying to keep the GNN code in sync between the releases.

The current way that we determine the fold for GN2 is a bit complicated and relies on a lot of reconstructed information. Only after we'd implemented this did we come up with something simpler: we just mix the jet rank (which enumerates the jets as clustered by fastjet) and the event number. The two together should give a unique address within a given simulated sample, so should be good for hashing.