21.9: Porting changes to fast clusterisation (as done in master) to profit of it by default.

Since I have updated the version for master (!36138 (merged)), I think would be useful to back-port it for 21.9, so that we can run the fast pixel clustering while preserving the output.

Changes in the RunUpgradeTests.py as the pixel clusters are sorted differently in ESDs and AODs. I have run IDPVM to compare the cluster quantities after running RunUpgradeTests.py using reference and patched versions and the results are 1:1 identical: https://test-dcube.web.cern.ch/test-dcube/dcube_test_clustering/