Move to const DataHandle to fix trigger ART tests (ATR-20597)

Hi,

this MR fixes the failing trigger ART tests in 21.3 (ATR-20597) by using const DataHandles in the muon converters. The change needs to be swept also to 21.9 but not to master (since there another technical approach is used). Marking the MR as urgent since ART tests are failing in current nightlies.

Adding @jchapman and @hartj

Best, Nico