Replace `graphene-django` with `Strawberry`

[ivarnakken]

Replace existing GraphQL implementation, which was built using graphene-django, with Strawberry.

graphene-django is unfortunately a dead project, and is no longer compatible with Django 4.0 and later releases (something which we currently work around via a monkey patch).

The goal here will be to keep the new GraphQL API as backward-compatible as possible, and to leverage any new features of Strawberry where they might be beneficial. Further research is needed to determine the migration strategy.

No changes to the database schema are anticipated.