build(deps): update sphinx requirement from <8 to <9 in /requirements (…

…#464)

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.rst)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v8.0.2)

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

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

requirements/requirements_doc.txt

@@ -1,4 +1,4 @@
-Sphinx<8
+Sphinx<9
 ansys-sphinx-theme==0.16.6
 imageio==2.34.2
 nest-asyncio==1.6.0