serializers-datacite: improve access rights

[slint]

Although somewhat nice to have a more explicit "access rights" value in the DataCite export, dumping info:eu-repo values under the rights field is a remnant from Zenodo/OpenAIRE integrations... In the OpenAIRE metadata guidelines, they require including this information, even though the entire info:eu-repo terms namespace is deprecated/defunct (the link I have in the commit message recently started to 404 in Dutch 🌷).

For repositories that want to be harvested by OpenAIRE via OAI-PMH this is a requirement and has to be exposed under the oai_datacite metadata format... Some better solutions I can think of to get out of this mess:

• Implement explicitly the OpenAIRE Guidelines as a separate export format exposed via OAI-PMH (there are other peculiarities as well)
• Use the COAR Access Rights vocabulary for our DataCite serialization, which I think maps nicely to our usage and is established/maintained. This could be enabled for everyone by default

Originally posted by @slint in #2047 (comment)