ATTESTATION.items can only referrer to EHR content

ATTESTATION is a common-type, used also by PARTY (demographic package), yet the specification is assuming (explicitly) that the 'items' attribute will target EHR content.

see 'items' in https://specifications.openehr.org/releases/RM/latest/common.html#_attestation_class.

To fix this, we will have to relax 'items' format by changing type to List<DV_URI> instead of List<DV_EHR_URI>.