Update docutils requirement from <0.18 to <0.21 (#3512)

Updates the requirements on [docutils](https://docutils.sourceforge.io/) to permit the latest version.

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

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

pyproject.toml

@@ -89,7 +89,7 @@ test = [
     "types-toposort"
 ]
 docs = [
-    "docutils<0.18",
+    "docutils<0.21",
     "sphinx>=5.3,<7.3",
     "sphinx_rtd_theme==1.2.0",
     # Regression on sphinx-autodoc-typehints 1.21