Bump plantuml-markdown from 3.9.0 to 3.9.1 in /CI (#1318)

Bumps [plantuml-markdown](https://github.com/mikitex70/plantuml-markdown) from 3.9.0 to 3.9.1.
- [Release notes](https://github.com/mikitex70/plantuml-markdown/releases)
- [Changelog](https://github.com/mikitex70/plantuml-markdown/blob/master/CHANGELOG.md)
- [Commits](mikitex70/plantuml-markdown@3.9.0...3.9.1)

---
updated-dependencies:
- dependency-name: plantuml-markdown
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

CI/requirements.txt

@@ -5,4 +5,4 @@ mkdocs-exclude==1.0.2
 mkdocs-redirects==1.2.0
 mkdocs-git-revision-date-localized-plugin==1.2.0
 mkdocs-minify-plugin==0.6.4
-plantuml-markdown==3.9.0
+plantuml-markdown==3.9.1