Replace TriggerFlags.EDMDecodingVersion with ConfigFlags.Trigger.EDMVersion and remove TriggerFlags.doMT

After !38381 (merged) we can now rely fully on the auto-configuration of EDMDecodingVersion from ConfigFlags.Trigger. Clean up the now-redundant old code. In addition, clean up all use-cases (some incorrect) of TriggerFlags.doMT and use other flags in its place.

Detailed changes:

1. Replace TriggerFlags.EDMDecodingVersion with ConfigFlags.Trigger.EDMDecodingVersion
   • Deprecate but keep TriggerFlags.EDMDecodingVersion and auto-configure it to the value of ConfigFlags.Trigger.EDMDecodingVersion
   • Replace all use cases of the old flag with the new one
   • Remove the old "auto" configuration function, which wasn't "auto" (it had to be called separately from the flag)
2. Rename EDMDecodingVersion to EDMVersion because it doesn't steer just decoding, but also encoding (i.e. defines what is added to the trigger output).
3. Remove TriggerFlags.doMT
   • Use ConfigFlags.Trigger.EDMVersion where the intention was to check for Trigger EDM version
   • Use ConfigFlags.Concurrency.NumThreads where the intention was to check for running AthenaMT vs serial athena

This touches many domains, so adding full-unit-tests

Closes ATR-22360

⚠ Please do not squash the three commits, they correspond to the three points above.