release abstracts mimetype uses less-popular JATS XML type

The current release importer code uses `application/xml+jats` for abstracts, but it seems that `application/jats+xml` may be more popular or "canonical".

At this point, with so many existing abstracts, I don't think it is worth updating all these. Should probably at least document in the guide though.

Start by locating the release importer and the guide referenced in the issue, then verify how the abstract MIME type is currently described. Update the guide to document the relevant JATS MIME type and the existing abstracts' status without changing historical records.

Written by the indexing model from the issue text.