Migrate to OpenAPI Schema Generation instead of CoreAPI

[andretheronsa]

Swagger/OpenAPI2.0 is becoming the standard for schema generation - I think it would be good for Geocontext to migrate from Legacy CoreAPI to a modern standard such as implimented by drf-yasg to ensure ongoing support. Support for CoreAPI will be dropped with 3.12.

DRF 3.10 announcement "Since we first introduced schema support in Django REST Framework 3.5, OpenAPI has emerged as the widely adopted standard for modeling Web APIs. This release begins the deprecation process for the CoreAPI based schema generation, and introduces OpenAPI schema generation in its place."
DRF 3.9 announcement
"We're planning to iteratively work towards OpenAPI becoming the standard schema representation. This will mean that the coreapi dependency will gradually become removed, and we'll instead generate the schema directly, rather than building a CoreAPI Document object. OpenAPI has clearly become the standard for specifying Web APIs, so there's not much value any more in our schema-agnostic document model. Making this change will mean that we'll more easily be able to take advantage of the full set of OpenAPI functionality. This will also make a wider range of tooling available."