[Security] Bump kramdown from 2.1.0 to 2.3.0

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.1.0 to 2.3.0. **This update includes a security fix.**
- [Release notes](https://github.com/gettalong/kramdown/releases)
- [Changelog](https://github.com/gettalong/kramdown/blob/master/doc/news.page)
- [Commits](https://github.com/gettalong/kramdown/commits)

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

Gemfile.lock

@@ -39,7 +39,8 @@ GEM
     highline (2.0.3)
     jaro_winkler (1.5.4)
     json (2.3.0)
-    kramdown (2.1.0)
+    kramdown (2.3.0)
+      rexml
     librariesio-gem-parser (1.0.0)
     libv8 (3.16.14.19)
     oga (2.15)
@@ -52,6 +53,7 @@ GEM
     rainbow (3.0.0)
     rake (13.0.1)
     ref (2.0.0)
+    rexml (3.2.4)
     rspec (3.9.0)
       rspec-core (~> 3.9.0)
       rspec-expectations (~> 3.9.0)