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.
- [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[bot] <support@github.com>

Gemfile.lock

@@ -34,7 +34,8 @@ GEM
       faraday (~> 0.8)
     git (1.6.0)
       rchardet (~> 1.8)
-    kramdown (2.1.0)
+    kramdown (2.3.0)
+      rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     multipart-post (2.1.1)
@@ -47,6 +48,7 @@ GEM
     public_suffix (4.0.3)
     rake (13.0.1)
     rchardet (1.8.0)
+    rexml (3.2.4)
     sawyer (0.8.2)
       addressable (>= 2.3.5)
       faraday (> 0.8, < 2.0)