build(deps): bump docutils from 0.20.1 to 0.21.1 in /lib/spack/docs

Bumps [docutils](https://docutils.sourceforge.io) from 0.20.1 to 0.21.1.

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

Signed-off-by: dependabot[bot] <support@github.com>

lib/spack/docs/requirements.txt

@@ -3,7 +3,7 @@ sphinxcontrib-programoutput==0.17
 sphinx_design==0.5.0
 sphinx-rtd-theme==2.0.0
 python-levenshtein==0.25.0
-docutils==0.20.1
+docutils==0.21.1
 pygments==2.17.2
 urllib3==2.2.1
 pytest==8.1.1