Ask IANA to update registration information

Per rfc8126 section 8 we should request that the registry information is updated to the new RFC. It should be enough to add a note like:

  Because this document obsoletes RFC 4637, IANA is asked to change
  all registration information that references [RFC4637] to instead
  reference [[this RFC]].

It's not clear how we should approach the existing "This specification creates a registry" part. See ml post.