`PUT` on a clarification via api/doc doesn't work

We use `formdata` in the `api/doc` to submit the `PUT` request but this is out of spec.

```
curl -X 'PUT' \
  'http://localhost/domjudge/api/v4/contests/demo/clarifications/1?strict=false' \
  -H 'accept: application/json' \
  -H 'Content-Type: multipart/form-data' \
  -F 'text=same text as string' \
  -F 'time=2025-10-26T13:38:47.878Z' \
  -F 'id=1'
```