build(deps): update markdownify requirement

Updates the requirements on [markdownify](https://github.com/matthewwithanm/python-markdownify) to permit the latest version.
- [Release notes](https://github.com/matthewwithanm/python-markdownify/releases)
- [Commits](matthewwithanm/python-markdownify@0.11.0...0.13.1)

---
updated-dependencies:
- dependency-name: markdownify
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements/base.txt

@@ -1,6 +1,6 @@
 imagesize>=1.4,<1.5
 lunr[languages]>=0.7,<0.8
-markdownify>=0.11,<0.13
+markdownify>=0.11,<0.14
 Mastodon.py>=1.8.1,<1.9
 orjson>=3.8,<3.11
 packaging>=20,<25