Remove MU15 and add MU21 to v7 L1 menu (ATR-21919)
The v7 menu in master was using MU15 and MU20 as the last two L1 muon thresholds, which is not consistent with the menu we have in master for the MT setup or the menu we used in 2018. I updated the menu to have MU20 and MU21 to be consistent, see https://its.cern.ch/jira/browse/ATR-21919