Update sphinx-sitemap requirement from >=2.6.0 to >=2.9.0

Updates the requirements on [sphinx-sitemap](https://github.com/jdillard/sphinx-sitemap) to permit the latest version.
- [Release notes](https://github.com/jdillard/sphinx-sitemap/releases)
- [Changelog](https://github.com/jdillard/sphinx-sitemap/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/jdillard/sphinx-sitemap/compare/v2.6.0...v2.9.0)

---
updated-dependencies:
- dependency-name: sphinx-sitemap
  dependency-version: 2.9.0
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -10,7 +10,7 @@ sphinx-autobuild>=2021.3.14
 sphinx-copybutton>=0.5.2
 sphinxext-opengraph>=0.9.0
 sphinx-autodoc-typehints>=1.25.0
-sphinx-sitemap>=2.6.0
+sphinx-sitemap>=2.9.0
 
 # Alternative modern theme (optional)
 furo>=2024.1.29