Build(deps): Bump rdoc from 6.7.0 to 6.8.1 (#29829)

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](https://github.com/ruby/rdoc/compare/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] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile.lock

@@ -399,7 +399,7 @@ GEM
       msgpack (>= 0.4.3)
       optimist (>= 3.0.0)
     rchardet (1.8.0)
-    rdoc (6.7.0)
+    rdoc (6.8.1)
       psych (>= 4.0.0)
     redcarpet (3.6.0)
     redis (4.8.1)