Merge 21.0.110 into 21.3

This merge request merges all the changes that were made to the 21.0 branch between the release release/21.0.110 and the previous release release/21.0.109.

Below is the full list of merge requests that were included between these two tags:

• !31125 (merged) : Adding ART tests for testing Tracking overlay
• !30863 (merged) : Change BCID structure to trees
• !30900 (merged) : Fix nhit calculation and debug output in TFCSLateralShapeParametrizationHitChain
• !30922 (merged) : Update TrackEventAthenaPool unit test reference files after changes in !28312 (merged).

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