Draft: spec: Updates DCC credential subject in docs to match model - Attestation.

Updates the DCC.credentialSubject to be an dcc.Attestation, since in !566 (merged) the DCC model was updated to match the pattern in DPP. But I've since noticed that the DCC.credentialSubject was already a ConformityAttestation in 0.5.0 - so I want to verify before moving ahead with this.

Background

For the DPP, we've now successfully updated the credentialSubject to be a Product (which is specifically a dpp.Product which inherits from the core.Product and adds other claim fields). This was to avoid having something other than a Product as the credentialSubject for a DPP.

In !566 (merged) , I did assume that we'd want the same for a DCC.credentialSubject: rather than creating a new ConformityAttestation type, the credentialSubject is an Attestation (which is specifically a dcc.Attestation which inherits from the core.Attestation and adds other claim/assessment fields).

But I realise now, that although that would provide consistency with the DPP, it may have been an incorrect assumption: unlike the DigitalProductPassport which in name would expect a Product as the credentialSubject, the DigitalConformityCredential name doesn't mention the word Attestation at all, which may be the reason for calling it the ConformityAttestation (a name it already had in 0.5.0). That said, using two different types (Attestation and ConformityAttestation) can mean that we confuse ids and concepts (similar to one of the reasons we moved from having a separate ProductPassport as the DPP credentialSubject, to just using a DPP Product).

So let me know @steve.capell - if that assumption was incorrect, I'll simply revert the change to the model (in jargon and here in the spec) so that the credentialSubject is once more a ConformityAttestation, and close this MR. If it was a correct assumption, I'll finish this MR (update the SVG) and mark it ready for review. Thanks!