[Bug] "poi_mins" not defined

The poi_mins is needed in plots/likelihoods.py when making plots, otherwise it always returns [None, None]

However, poi_mins is not defined in tasks/likelihoods.py

Need to change poi_min to poi_mins in 2 places:

• (1) PlotMultipleLikelihoodScans https://gitlab.cern.ch/hh/tools/inference/-/blob/master/dhi/tasks/likelihoods.py?ref_type=heads#L626
• (2) PlotMultipleLikelihoodScansByModel https://gitlab.cern.ch/hh/tools/inference/-/blob/master/dhi/tasks/likelihoods.py?ref_type=heads#L782