Bump rdoc from 6.8.1 to 6.9.0

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.8.1 to 6.9.0.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.8.1...v6.9.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -235,7 +235,7 @@ GEM
       zeitwerk (~> 2.6)
     rainbow (3.1.1)
     rake (13.2.1)
-    rdoc (6.8.1)
+    rdoc (6.9.0)
       psych (>= 4.0.0)
     regexp_parser (2.9.3)
     reline (0.5.12)