[HTTP] Implement Extensible Prioritization Scheme for HTTP/2

[hamishwillee]

Acceptance criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes for enabled/preview features
• Add/remove entry to Firefox experimental features page for preference/released features

Features to document

I am not entirely sure what is relevant and what needs to be documented. However the spec indicates a new Priority HTTP header (possibly HTTP3 only) in https://www.rfc-editor.org/rfc/rfc9218.html#section-5 and we generally document those.

This also disables the previous HTTP/2 approach defined in network.http.http2.enabled.deps which does not appear to have any "public" headers.

We also would probably add the spec to https://developer.mozilla.org/en-US/docs/Web/HTTP/Resources_and_specifications

FYI this is enabled by default

• network.http.priority_header.enabled - enables new Priority header from RFC9218
• network.http.http2.enabled.deps - turns off old RFC7540 stream dependencies

More investigation is needed, and there is a little background here: https://blog.cloudflare.com/adopting-a-new-approach-to-http-prioritization/

Related Gecko bugs

1865040: Implement Extensible Prioritization Scheme for HTTP/2

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.

[hamishwillee]

Status

• BCD FF128 HTTP Priority - RFC9218 Extensible Prioritization Scheme for HTTP browser-compat-data#23511
• Docs HTTP Priority header #34399
• Release notes FF128 Relnote: RFC 9218: Extensible Prioritization Scheme for HTTP #34474
• dev-doc-complete