build(deps): update markdown requirement in /hack/mkdocs/image (#4126)

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](Python-Markdown/markdown@3.8...3.9.0)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: k8s-infra-cherrypick-robot

hack/mkdocs/image/requirements.txt

@@ -5,7 +5,7 @@ jsmin==3.0.1
 livereload==2.7.1
 # mkdocs 2.4.1 requires Markdown < 3.4.0
 # https://github.com/kubernetes-sigs/gateway-api/pull/1671#issuecomment-1400586465
-markdown~=3.8
+markdown~=3.9
 MarkupSafe==3.0.2
 mkdocs==1.6.1
 mkdocs-awesome-pages-plugin==2.10.1