Update rio_api requirement from 0.5 to 0.6

Updates the requirements on [rio_api](https://github.com/oxigraph/rio) to permit the latest version.
- [Release notes](https://github.com/oxigraph/rio/releases)
- [Changelog](https://github.com/oxigraph/rio/blob/master/CHANGELOG.md)
- [Commits](https://github.com/oxigraph/rio/compare/v0.5.0...v0.6.0)

---
updated-dependencies:
- dependency-name: rio_api
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

lib/Cargo.toml

@@ -33,7 +33,7 @@ digest = "0.9"
 regex = "1"
 oxilangtag = "0.1"
 oxiri = "0.1"
-rio_api = "0.5"
+rio_api = "0.6"
 rio_turtle = "0.5"
 rio_xml = "0.5"
 hex = "0.4"