HTTP/2: React to updated spec (RFC 9113)

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

New RFC dropped. Differences from the earlier spec: https://www.rfc-editor.org/rfc/rfc9113.html#appendix-B

@Tratcher @davidfowl

### Describe the solution you'd like

Look through changes. We don't implement dropped features (prioritization, HTTP 1.1 upgrade) but they tighten and clarify validations rules.

We could follow the updated spec or stay as we are to maximize compatibility.

### Additional context

_No response_