Update sphinx requirement from <7.4,>=7 to >=7,<8.1

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/v8.0.2/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.0.0...v8.0.2)

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

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

docs/requirements.txt

@@ -1,4 +1,4 @@
 furo==2024.5.6
 myst-parser[linkify]>=2,<4
-sphinx>=7,<7.4
+sphinx>=7,<8.1
 sphinx-copybutton>=0.3.1,<1