Update monGroup for tau20/25 mediumGNTau cTAU chains

This MR is adding histograms in online tau trigger monitoring for the following two chains: HLT_tau20_mediumGNTau_L1cTAU20M and HLT_tau25_mediumGNTau_L1cTAU20M. This will help in validation of the GNTau online TauID

Tagging @smwang and @slai