Update receipts header parameter references

[OR13]

Assuming cose-wg/draft-ietf-cose-merkle-tree-proofs#18 is merged, that document will register everything that is needed for SCITT Receipts (a profile of COSE Receipts).

There is clean up work to do in the architecture.

• Remove the editorial notes regarding 394.
• Review IANA considerations (AFAIK, 394 was never requested for registration in the document, despite the editor notes).

[OR13]

I've merged the PR, but we need to publish a new version to the data tracker.

Ideally other authors (of COSE Receipts / COMETRE) will do an editorial pass, and then push a new version to the data tracker.