Disable the possibility to run RDOtoRDOTrigger with serial athena

Legacy (Run-2) Trigger code is about to be removed from the master branch. As the first step (ATR-22725), simply disable the possibility to run it in the RDOtoRDOTrigger transform. The actual code removal will be done in next steps.

Changes:

1. Remove the inclusion of the legacy skeleton from the main one and instead throw an error if a job attempts to run legacy RDOtoRDOTrigger.
2. Update RunTier0Tests.py to use the Run-3 Trigger for RDOtoRDOTrigger step in q221 and q440 (enabled by threads=1).

See also the presentation in 2 February Reconstruction meeting:
https://indico.cern.ch/event/1003611/contributions/4214559/attachments/2182829/3687677/legtrigremove_reco.pdf

The CI has already been updated to stop running the legacy Trigger: https://gitlab.cern.ch/atlas-sit/CI/-/merge_requests/202