Merge release/21.0.92 into 21.3 branch

This merge request merges the changes that went into the 21.0 branch between the release/21.0.91 and release/21.0.92 tags into the 21.3 branch.

release/21.0.92 contains the following frozen-tier0-violating merge requests:

• athenaprivate1!15481
• athenaprivate1!16537

MRs in https://gitlab.cern.ch/atlas/athena/tags/nightly/21.0/2019-01-06T2159, accepted since https://gitlab.cern.ch/atlas/athena/tags/nightly/21.0/2018-12-13T2202:

• athenaprivate1!16769: FW-like Code Compatibility
• athenaprivate1!15481: Sweeping athenaprivate1!15419 from 21.2 to 21.0. Fix MET_Truth muon selections for IntOut and IntMuons terms
• athenaprivate1!16537: Merge 21.1 into 21.0

The full diff between release/21.0.92 and release/21.0.91 is available at https://gitlab.cern.ch/atlas/athena/compare/release/21.0.91...release/21.0.92