[RTA/DPA BW tests]: Minor bandwidth test maintenance

FYI @jconnaug @lugrazet

Should be merged with PRConfig!568 and lhcb-core/LHCbPR2HD!316 (merged) just before the nightlies.

Example webpages at https://rjhunter-bandwidth.web.cern.ch/Moore4975/

A handful of improvements:

• Print bandwidths in general in MB/s rather than GB/s as requested by the PPG,
• (to make the above less painful) Store our dataframe column titles in a little static helper class - we had loads of e.g. "Total Bandwidth (GB/s)" hard-coded strings across our analysis scripts as they have grown. This is now defined only once. Headline bar charts should still be in GB/s though as that is the appropriate unit there,
• Reduce the threshold for a line's bandwidth to be highlighted in red to 100 MB/s. Very few lines should take more than that,
• Rename hlt2_bandwidth_input_2024.yaml -> hlt2_bandwidth_input.yaml - there's no need to append the year to it, we have git,
• Prevent possible race condition in the qmtests coming from read_event_numbers.py.

TODO:

• Local test and inspect the pages for MB/s vs GB/s booboos.