Retire TwoStage regridder?

# ✨ Feature Request

## Requirements
Iris-esmf-regrid now has [​support for non-degree](https://github.com/SciTools-incubator/iris-esmf-regrid/pull/178) coordinate reference systems. This gives us a path to retire the TwoStage regridder.

This would mean replacing existing regridding calls to the TwoStage regridder with calls to the iris-esmf-regrid conservative regridder.

It would be good to take the opportunity to replace the existing calls to the iris linear and area weighted regridders with calls to the iris-esmf-regrid equivalents at the same time. This would align regular ANTS with UG-ANTS too.

A big advantage is that this would imply that a lot of our existing coordinate reference system could be retired - the only parts we would need would be the definitions of equivalent CRSs (e.g. treating WGS84 and UM_SPHERE interchangeably).

This feels like a sufficiently breaking change that it should be part of ANTS 4.x?
## Context

Care will need to be taken to check how this interacts with the decomposition.

Start by locating existing calls to the TwoStage, Iris linear, and area-weighted regridders, then trace how those calls interact with decomposition. Review the iris-esmf-regrid non-degree CRS support and determine the scope of the breaking change; done means the relevant calls use the equivalent iris-esmf-regrid regridders and unnecessary CRS definitions are retired.

Written by the indexing model from the issue text.