More generic identifier wording

Created by: onthebreeze

In https://spec-untp-fbb45f.opensource.unicc.org/docs/about/Requirements#compatibility--interoperability-requirements, reworded CI.06 and CI.07 to be more generic about re-use of existing identifier schemes.

In https://spec-untp-fbb45f.opensource.unicc.org/docs/specification/DigitalTraceabilityEvents, ensured consistent reference to ISO/IEC 19987 and used more generic scheme identifiers in data snippets.

In https://spec-untp-fbb45f.opensource.unicc.org/docs/about/References also rewoded to ensure consistent reference to ISOIEC 19987