Bump kramdown from 2.3.2 to 2.4.0

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.3.2 to 2.4.0.
- [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)

---
updated-dependencies:
- dependency-name: kramdown
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -216,7 +216,7 @@ GEM
       activerecord
       kaminari-core (= 1.2.2)
     kaminari-core (1.2.2)
-    kramdown (2.3.2)
+    kramdown (2.4.0)
       rexml
     listen (3.7.1)
       rb-fsevent (~> 0.10, >= 0.10.3)