feat: replace `django-rest-swagger` with `drf-spectacular`

[afuetterer]

Description

Related issue: #698

Tasks

• [x] decide about NOTICE
• [x] remove swagger.scss
• [ ] decide about the schema
• [ ] decide about the purpose of the static schema in https://github.com/rdmorganiser/rdmo-openapi, which is 6 years old
• [ ] handle warning and errors in schema generation
• [ ] add tests
• [ ] delete temporary django-rest-swagger.yaml schema file
• [ ] remove ignore warnings from pytest config
• [ ] squash commits

Types of Changes

Mix between bug fix (deprecated library is replaced), new feature (schema is enhanced) and maybe breaking change (schema is different from before).

Checklist

• [x] I have read the contributor guide.
• [x] My code follows the code style of this project.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

Refs

• https://drf-spectacular.readthedocs.io/en/latest/settings.html
• https://drf-spectacular.readthedocs.io/en/latest/faq.html#i-get-a-lot-of-warnings
• https://www.django-rest-framework.org/api-guide/versioning/#urlpathversioning