Bump kramdown from 1.17.0 to 2.3.1

Bumps [kramdown](https://github.com/gettalong/kramdown) from 1.17.0 to 2.3.1.
- [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: indirect
...

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Author: dependabot[bot]