Specify encoding of ECDSA signatures

[letmaik]

The spec currently defines ES256 and references RFC6979. However, that RFC doesn't say anything about the encoding of the two signature components as that's typically left to the container format.

COSE makes this concrete in https://www.rfc-editor.org/rfc/rfc9053.html#name-ecdsa.

I think changing the reference to section 2.1. of RFC 9053 "CBOR Object Signing and Encryption (COSE): Initial Algorithms" would do the trick.