Update sphinx requirement from <5.0.0 to <8.0.0

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/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v2.0.0...v7.2.6)

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

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

docs/requirements.txt

@@ -8,7 +8,7 @@ requests>=2.21.0
 
 # Building Docs requirements
 sphinxjp.themes.basicstrap
-sphinx<5.0.0
+sphinx<8.0.0
 sphinx-gallery>0.6.2
 pillow>=7.1.2
 breathe>=4.34.0