This merge request merges all the changes that were made to the ~"21.0" branch between the release
[release/21.0.110](https://gitlab.cern.ch/atlas/athena/tags/release/21.0.110) and the previous release
[release/21.0.109](https://gitlab.cern.ch/atlas/athena/tags/release/21.0.109).

Below is the full list of merge requests that were included between these two tags:
 * !31125 : Adding ART tests for testing Tracking overlay
 * !30863 : Change BCID structure to trees
 * !30900 : Fix nhit calculation and debug output in TFCSLateralShapeParametrizationHitChain
 * !30922 : Update TrackEventAthenaPool unit test reference files after changes in !28312.

Link to the full diff between [release/21.0.110](https://gitlab.cern.ch/atlas/athena/tags/release/21.0.110) and
[release/21.0.109](https://gitlab.cern.ch/atlas/athena/tags/release/21.0.109)
is available at
https://gitlab.cern.ch/atlas/athena/compare/release/21.0.109...release/21.0.110