Release notes for release/21.0.112

The release release/21.0.112 was built from the tag nightly/21.0/2020-04-30T2147

This is the list of merge requests that were included since the previous release release/21.0.111:

Link to the full diff between release/21.0.112 and release/21.0.111 is available at https://gitlab.cern.ch/atlas/athena/compare/release/21.0.111...release/21.0.112