Change filter output to mDST to be consistent with S21

The S21r1 filter had the same issue as the S21 once had (https://gitlab.cern.ch/lhcb-datapkg/WG/RDConfig/-/blob/master/options/FilterLb2NstGamma-Stripping21.py). The output is set to DST when I actually want mDST.

The proper mDST writers are called now, with some other changes within the file triggered by the autoformatter.

Tested it against evt+std://MC/2011/11102202/Beam3500GeV-2011-MagDown-Nu2-Pythia8/Sim09b/Trig0x40760037/Reco14c/Stripping21r1NoPrescalingFlagged/ALLSTREAMS.DST. It provides a 4% efficiency from it with mDST output: 000000.Lb2NstGamma.Strip.mdst. All as expected