chore: add explicit `ConformityAssessment.conformance` after removing from base class.

Created by: un-cefact-github-app[bot]

Add explicit ConformityAssessment.conformance boolean after removing the property from the Declaration base class. See https://github.com/uncefact/spec-untp/issues/415

As this is the first PR for this domain post 0.6.1, it includes:

• Switching to working versions of core and ConformityCredential
• Removal of the encryptionMethod property from working core

The actual addition of conformance looks like a move since it was being defined by the base class.