Bump rdf from 3.1.5 to 3.1.6 (#14736)

Bumps [rdf](https://github.com/ruby-rdf/rdf) from 3.1.5 to 3.1.6.
- [Release notes](https://github.com/ruby-rdf/rdf/releases)
- [Changelog](https://github.com/ruby-rdf/rdf/blob/develop/CHANGES.md)
- [Commits](https://github.com/ruby-rdf/rdf/compare/3.1.5...3.1.6)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -476,7 +476,7 @@ GEM
       thor (>= 0.19.0, < 2.0)
     rainbow (3.0.0)
     rake (13.0.1)
-    rdf (3.1.5)
+    rdf (3.1.6)
       hamster (~> 3.0)
       link_header (~> 0.0, >= 0.0.8)
     rdf-normalize (0.4.0)