Place fields in correct IANA Registry

[CxRes]

The header fields must be registered in the Hypertext Transfer Protocol (HTTP) Field Name Registry https://www.iana.org/assignments/http-fields/ and not the "Message Headers" registry https://www.iana.org/assignments/message-headers/ as was the case in days of yore.

See updated practice here https://www.rfc-editor.org/rfc/rfc9110.html#name-field-name-registration.