Exported Workflow RO-Crates do not include datePublished in root data entity, only sdDatePublished

[elichad]

Describe the bug
I downloaded this workflow as an RO-Crate and tested it with a dev version of https://github.com/crs4/rocrate-validator, which flagged that datePublished is missing from the root data entity.

From the spec https://www.researchobject.org/ro-crate/specification/1.1/root-data-entity.html#direct-properties-of-the-root-data-entity:

The Root Data Entity MUST have the following properties:

• ...
• datePublished: MUST be a string in ISO 8601 date format and SHOULD be specified to at least the precision of a day, MAY be a timestamp down to the millisecond.
• ...

Expected behavior
The Root Data Entity of exported RO-Crates should include datePublished indicating the publication date of the current version of the workflow, as well as the existing sdDatePublished field indicating when the RO-Crate metadata file was generated.