Decouple release tags from public XML

[jcoyne]

In order to support versioning, where a public XML file will represent a user version, we need to remove the release tags from public XML. The release tags are not associated with any particular version and have been moved to a separate model in SDR.

See

purl-fetcher/app/services/purl_cocina_updater.rb

Line 44 in 2647153

active_record.refresh_release_tags(releases)

This is currently blocked by #687

[andrewjbtw]

Correct me if I"m wrong, but I think we accomplished this with the meta_json holding the release info.