build(deps): bump pygments from 2.17.2 to 2.18.0 in /lib/spack/docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.17.2 to 2.18.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.17.2...2.18.0)

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

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

lib/spack/docs/requirements.txt

@@ -4,7 +4,7 @@ sphinx_design==0.5.0
 sphinx-rtd-theme==2.0.0
 python-levenshtein==0.25.1
 docutils==0.21.2
-pygments==2.17.2
+pygments==2.18.0
 urllib3==2.2.1
 pytest==8.2.0
 isort==5.13.2