Add `bunchgroupset` to the list of allowable trigger menu JSON file types when peeking the metadata

This reacts to an observation in the CI from !60495 (merged)

Py:MetaReader     WARNING Got an xAOD::TriggerMenuJson called bunchgroupset but only expecting hltmenu or l1menu

bunchgroupset is a valid identifier, albeit one which was added quite late. Hence being missing here.