Mark ListElementEntities as deprecated in OpenAPI docs

Erwan Rouchet
2022-07-12T15:13:52Z

Use the `deprecated` argument of [extend_schema](https://drf-spectacular.readthedocs.io/en/latest/drf_spectacular.html#drf_spectacular.utils.extend_schema) to mark `ListElementEntities` as disappearing soon.

This should show up in `/api...