ATLASDPD-2220: Merge of !77747 and !77764 to work around reference file updates

Apparently it's difficult to make two changes to the reference files at once. This hopefully simplifies the problem by merging

• !77747 (closed) : jet energy regression with a transformer
• !77764 (closed) : GN3 flavor tagging algorithm

into one MR. Pinging @tsulaia, and marking urgent since this was supposed to go into the PHYS production release.