Rename LFRic vertical namelist handling in regular ANTS

# ✨ Feature Request
The UM requires some specific manipulation of the data loaded from vertical namelists to merge the 0th and 1st levels into a single level. We need to keep this.

LFRic does not require this manipulation. While we had the prototype UGrid regridding in regular ANTS, vertical namelist support without the UM workaround was added and called [​load_lfric_vertical](https://github.com/MetOffice/ANTS/blob/06fdecaeb3850d06cb3f2e50c6d12cae2040843a/lib/ants/fileformats/namelist/__init__.py#L129). We should rename this - it may still be useful as part of using ANTS as a preprocessor for UG-ANTS, but the current name implies LFRic support in regular ANTS.

The equivalent [load_um_vertical](https://github.com/MetOffice/ANTS/blob/06fdecaeb3850d06cb3f2e50c6d12cae2040843a/lib/ants/fileformats/namelist/__init__.py#L103) may need a rename for consistency too.