Update sphinx requirement from <7.0,>=6.0 to >=6.0,<9.0

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v6.0.0...v8.1.3)

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

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

Author: dependabot[bot]

_requirements/docs.txt

@@ -1,4 +1,4 @@
-sphinx >=6.0,<7.0
+sphinx >=6.0,<9.0
 myst-parser >=2.0.0
 nbsphinx >=0.8.5
 pandoc >=1.0