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: direct:development
  update-type: version-update:semver-minor
...

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

Gemfile

@@ -63,7 +63,7 @@ end
 group :test do
     gem 'test-unit-full', '~> 0.0.5'
     gem 'rspec', '~> 3.13'
-    gem 'rdoc', '~> 6.7'
+    gem 'rdoc', '~> 6.8'
     gem 'browserstack-local', '~> 1.4'
 
     gem 'irb', '~> 1.14'

Gemfile.lock

@@ -138,7 +138,7 @@ GEM
     pry-byebug (3.10.1)
       byebug (~> 11.0)
       pry (>= 0.13, < 0.15)
-    psych (5.1.2)
+    psych (5.2.0)
       stringio
     public_suffix (6.0.1)
     qr4r (0.6.2)
@@ -153,7 +153,7 @@ GEM
       rack (>= 1.3)
     rainbow (3.1.1)
     rake (13.2.1)
-    rdoc (6.7.0)
+    rdoc (6.8.1)
       psych (>= 4.0.0)
     regexp_parser (2.9.2)
     reline (0.5.10)
@@ -216,7 +216,7 @@ GEM
     sqlite3 (2.2.0-x86-linux-gnu)
     sqlite3 (2.2.0-x86_64-darwin)
     sqlite3 (2.2.0-x86_64-linux-gnu)
-    stringio (3.1.1)
+    stringio (3.1.2)
     sync (0.5.0)
     term-ansicolor (1.11.2)
       tins (~> 1.0)
@@ -297,7 +297,7 @@ DEPENDENCIES
   rack (~> 2.2)
   rack-protection (~> 3.2.0)
   rake (~> 13.2)
-  rdoc (~> 6.7)
+  rdoc (~> 6.8)
   rest-client (~> 2.1.0)
   rspec (~> 3.13)
   rubocop (~> 1.68.0)