Make credential succession checking the AS's job

[kkohbrok]

Right now it says in the Update proposal section: "Verify that the set of identities attested by the credential is acceptable to the application for the participant being updated."

Firstly, I think this is a matter of authentication and thus the AS' job and secondly, it's a validation step that should probably go into the KeyPackage Validation section.

[bifurcation]

Discussion on virtual interim:

• Depends on what the boundary of the AS is:
  • Checking credential is valid
  • Also checking (credential -> credential) transitions
• Moving this check to KP validation section would make it clearer
• @kkohbrok to make a PR

[bifurcation]

Fixed by #584