Merging step is incorrect for DST output

Currently the merging step only supports ntuples:

https://gitlab.cern.ch/lhcb-dpa/analysis-productions/LbAnalysisProductions/-/blob/e4ec361bcacc9dcda03363d3a8e67f6d8aba227d/src/LbAnalysisProductions/submission.py#L365-380

This should be fixed to also suport DST.

See https://lblogbook.cern.ch/Operations/37252