Section 3.1: Hyperlinks cite obsolete RFC 7231 instead of current RFC 9110

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In this document, it will specifically cover proactive negotiation using the Accept header.

RFC 7231 is obsoleted by RFC 9110.

The current guidance regarding Content Negotiation and the Accept header is:

• https://datatracker.ietf.org/doc/html/rfc9110#name-proactive-negotiation
• https://datatracker.ietf.org/doc/html/rfc9110#field.accept