MNT: (deps): Bump sphinx from 5.2.3 to 8.1.3 in /ci

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 5.2.3 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v5.2.3...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

ci/doc_requirements.txt

@@ -1,7 +1,7 @@
 cartopy==0.24
 matplotlib==3.9.2
 metpy==1.6.3
-sphinx==5.2.3
+sphinx==8.1.3
 sphinx_rtd_theme==1.0.0
 sphinx-gallery==0.18.0
 myst-parser==0.18.1