MC21 configuration for the 2022 low mu run (420000)

Cleaned-up configurations for ATLMCPROD-10458.

PreInclude for Simulation (with this then the DataRunNumber setting is no longer required): Campaigns/MC21SimulationLowMuRun.py

PreInclude for Digitization (with this then the preExec to set digitizationFlags.dataRunNumber is no longer required): Campaigns/MC21NoPileUpLowMuRun.py

Tagging @kkrizka