BREAKING CHANGE: eliminate `rdflib.term.Genid` and `rdflib.term.RDFLibGenid`

[aucampia]

Summary of changes

• Added docstring to Graph.de_skolemize method
• Added a new test to test when a uriref parameter is passed to the de_skolemize method
• Fixes https://github.com/RDFLib/rdflib/issues/1828

Checklist

• [ ] Checked that there aren't other open pull requests for the same change.
• [ ] Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • [ ] Created an issue to discuss the change and get in-principle agreement.
  • [ ] Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • [ ] Added or updated tests that fail without the change.
  • [ ] Updated relevant documentation to avoid inaccuracies.
  • [ ] Considered adding additional documentation.
• [ ] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.