Update sphinx requirement from <6 to <8 in /requirements

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/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v7.0.0)

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

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

requirements/requirements_docs.txt

@@ -1,4 +1,4 @@
-Sphinx<7
+Sphinx<8
 ansys-sphinx-theme==0.9.8
 imageio==2.28.1
 notfound==1.0.2