docs(http): Update link to IANA HTTP fields registry

[bsmth]

Description

Updating the URL for IANA HTTP field registry.

Motivation

New document address for https://www.iana.org/assignments/message-headers/message-headers.xhtml#prov-headers

HTTP field name registrations have been moved to [https://www.iana.org/assignments/http-fields]
per [RFC-ietf-httpbis-semantics-19].

Related issues and pull requests

Fixes #32453

[github-actions]

Preview URLs

• /en-US/docs/MDN/Writing_guidelines/Howto/Document_an_HTTP_header
• /en-US/docs/Web/HTTP/Headers
• /en-US/docs/Web/HTTP

External URLs (8)

URL: /en-US/docs/MDN/Writing_guidelines/Howto/Document_an_HTTP_header
Title: How to document an HTTP header

• https://en.wikipedia.org/wiki/List_of_HTTP_header_fields (1 time) (Note! This may be a new URL 👀)
• https://www.iana.org/assignments/http-fields/http-fields.xhtml (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Web/HTTP
Title: HTTP

• https://github.com/protocol-registries/http-fields?tab=readme-ov-file (1 time) (Note! This may be a new URL 👀)
• https://www.iana.org/assignments/http-fields/http-fields.xhtml (1 time) (Note! This may be a new URL 👀)

---

URL: /en-US/docs/Web/HTTP/Headers
Title: HTTP headers

• https://datatracker.ietf.org/doc/html/rfc4229 (1 time) (Note! This may be a new URL 👀)
• https://datatracker.ietf.org/doc/html/rfc6648 (1 time) (Note! This may be a new URL 👀)
• https://github.com/protocol-registries/http-fields?tab=readme-ov-file (1 time) (Note! This may be a new URL 👀)
• https://www.iana.org/assignments/http-fields/http-fields.xhtml (2 times) (Note! This may be a new URL 👀)

[hamishwillee]

Looks good to me thanks!