update refs for HTTP Message Signatures & Digest Fields

[Brian Campbell]

HTTP Message Signatures and Digest Fields have just been published as RFCs 9421 & 9530 so the references in FAPI 2.0 Message Signing can be updated.

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https://mailarchive.ietf.org/arch/msg/rfc-dist/5zaFde1ttqND9S3DyirblYNd9ow/

https://www.rfc-editor.org/info/rfc9421

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https://mailarchive.ietf.org/arch/msg/rfc-dist/NHcS-Oj7i1mxddkUzTYcT1dQz28/

https://www.rfc-editor.org/info/rfc9530