Confirm JSON Schema version for PURL schemas

[mjherzog]

The current proposed JSON schemas in #514 are set to:

"$schema": "http://json-schema.org/draft-07/schema#",

draft-07 is the version used for CycloneDX.

There are recommendations to upgrade to https://json-schema.org/draft/2020-12. There are also concerns about support for the newer json-schema version in open source tools.

This issue is intended to document the choice of the json-schema version for the PURL v1.0 Ecma Standard

[mjherzog]

Closed as duplicate of #518