Bump rdoc from 6.7.0 to 6.8.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.7.0 to 6.8.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.7.0...v6.8.1)

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

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

Gemfile.lock

@@ -261,7 +261,7 @@ GEM
       thor (~> 1.0, >= 1.2.2)
       zeitwerk (~> 2.6)
     rake (13.2.1)
-    rdoc (6.7.0)
+    rdoc (6.8.1)
       psych (>= 4.0.0)
     redis (5.3.0)
       redis-client (>= 0.22.0)