build(deps): bump urllib3 from 2.3.0 to 2.4.0 in /lib/spack/docs (#50014)

Bumps [urllib3](https://github.com/urllib3/urllib3) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](https://github.com/urllib3/urllib3/compare/2.3.0...2.4.0)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-version: 2.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

lib/spack/docs/requirements.txt

@@ -5,7 +5,7 @@ sphinx-rtd-theme==3.0.2
 python-levenshtein==0.27.1
 docutils==0.21.2
 pygments==2.19.1
-urllib3==2.3.0
+urllib3==2.4.0
 pytest==8.3.5
 isort==6.0.1
 black==25.1.0