Support RFC 9537

[RFC 9537 adds `redacted` to `rdapConformance`.](https://www.rfc-editor.org/rfc/rfc9537.html#name-rdap-conformance) This is currently unsupported by the `rdap-conformance` tool.

```
entity.standard,http://rdap:8000/entity/LISTER,failure,content,attribute is not permitted here or is non-standard and does not contain an underscore,"redacted",rfc7483,2.1
```