Change JSON schema for SCOAP

[ErnestaP]

The jsonschema library starting from 4.18 introduced the referencing for resolving $ref keyword. However, I noticed that titles should be self-describable: we have:

    "titles": {
            "$ref": "http://repo.scoap3.org/schemas/elements/titles.json"
    }

and should be:

    "titles": {
            "self":{
                "$ref": "http://repo.scoap3.org/schemas/elements/titles.json"
            }
        }

[ErnestaP]

We would need to adapt this change because Airflow, starting from 2.6.3 has jsonschema==4.18.0