TriggerJobOpts: roll back to v5 global conditions tag in HLT for Run3 MC

Hello,

This MR is temporarily changing the global conditions tag used by default in the HLT for Run3 MC, due to an incompatibility between MC21 RDO used in trigger ART tests and v6 MDT conditions (ATR-25570). It would be great if this MR could be merged asap to avoid interferences with upcoming MRs changing trigger counts.

Cheers, Bertrand