standardise_mom6_filenames.sh doesn't support non-annual filenames

[dougiesquire]

The script standardise_mom6_filenames.sh only removes underscores before the first instance of four consecutive digits. Aside from being somewhat unrobust, this also doesn't support non-annual files, e.g. *_2024_01.nc.

We could also extend the script to support OM2 output so the same script can be used in both places