Bump rdoc from 6.4.0 to 6.6.3.1

Bumps [rdoc](https://github.com/ruby/rdoc) from 6.4.0 to 6.6.3.1.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.4.0...v6.6.3.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-type: indirect
...

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

Gemfile.lock

@@ -330,7 +330,7 @@ GEM
       pry (>= 0.13, < 0.15)
     pry-rails (0.3.9)
       pry (>= 0.10.4)
-    psych (4.0.6)
+    psych (5.1.2)
       stringio
     public_suffix (5.0.0)
     puma (5.6.8)
@@ -405,7 +405,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rdoc (6.4.0)
+    rdoc (6.6.3.1)
       psych (>= 4.0.0)
     react_on_rails (12.0.1)
       addressable
@@ -572,7 +572,7 @@ GEM
       activesupport (>= 5.2)
       sprockets (>= 3.0.0)
     ssrf_filter (1.1.2)
-    stringio (3.0.2)
+    stringio (3.1.0)
     terser (1.1.14)
       execjs (>= 0.3.0, < 3)
     thor (1.3.1)