Fix a10t JetCollection name in Legacy JetMonitoring Tool

This MR addresses the issue with a10t chains in the legacy JetMonitoring tool observed in https://its.cern.ch/jira/browse/ATR-22590

We adopt the new name for the underlying jet collection of a10t chains (PtFrac5 --> PtFrac4) for processing new AthenaMT samples, and the plots are filled properly again.

Tagging @jbossios (btw, sorry for the many included commits, I don't know why they still show up after so many merges)