Update markdown requirement from ~=3.8 to ~=3.9

Updates the requirements on [markdown](https://github.com/Python-Markdown/markdown) to permit the latest version.
- [Release notes](https://github.com/Python-Markdown/markdown/releases)
- [Changelog](https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md)
- [Commits](https://github.com/Python-Markdown/markdown/compare/3.8...3.9.0)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: '3.9'
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -24,7 +24,7 @@ mkdocs-autolinks-plugin
 
 # Requirements for core
 jinja2~=3.1
-markdown~=3.8
+markdown~=3.9
 mkdocs-material-extensions~=1.3
 pygments~=2.19
 pymdown-extensions~=10.16