build(deps-dev): update rdoc requirement from 6.8.1 to 6.9.0

Updates the requirements on [rdoc](https://github.com/ruby/rdoc) to permit the latest version.
- [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: direct:development
...

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

Gemfile

@@ -34,5 +34,5 @@ end
 # `bundle config set --local without rdoc`
 # Then re-run `bundle install`.
 group :rdoc do
-  gem "rdoc", "6.8.1"
+  gem "rdoc", "6.9.0"
 end