Default netCDF fileformat?

At present, our output netCDFs default to `netCDF4-classic`. Should this be `netCDF4`?

Worth considering that ANTS netCDFs are (typically) only loaded via ANTS or via UKCA - the rest of the UM only loads F03 ancillary files derived from the netCDF files. UKCA has a bespoke saver, so does not need to use the same format as the default netCDF saver.

Suspect this could be an easy change to make, but it may be a tricky one to validate with all users.

Also worth considering (either in this issue, or a spin off issue) whether we should be enabling netCDF compression.

Start by locating the default netCDF saver used for ANTS netCDF outputs and compare its format handling with the bespoke UKCA saver. Check how the resulting files are consumed through ANTS, UKCA, and the F03 ancillary workflow. Done means reaching a documented decision on the default format, validation coverage, and whether compression belongs in this issue or a follow-up.

Written by the indexing model from the issue text.