Sweeping !55617 from master to 22.0. xAODEventFormat+xAODEventFormatCnv: Prepare for possible migration to 64-bit sgkey_t.

Atlas Nightlybuildrequested to merge
cherry-pick-b2cce8f5b71-22.0 into 22.0

xAODEventFormat+xAODEventFormatCnv: Prepare for possible migration to 64-bit sgkey_t.

See merge request !55617 (merged)

Merge request reports