Update sphinx requirement from ~=7.4 to ~=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/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.4.0...v8.1.3)

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

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

requirements.txt

@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: 2021 Open Networking Foundation <[email protected]>
 # SPDX-License-Identifier: Apache-2.0
 
-Sphinx~=7.4
+Sphinx~=8.1
 doc8~=1.1.2
 reuse~=4.0.3
 sphinx-rtd-theme~=3.0.1