Chore(deps): Update validate-pyproject requirement from <0.20 to <0.21

Updates the requirements on [validate-pyproject](https://github.com/abravalheri/validate-pyproject) to permit the latest version.
- [Release notes](https://github.com/abravalheri/validate-pyproject/releases)
- [Changelog](https://github.com/abravalheri/validate-pyproject/blob/main/CHANGELOG.rst)
- [Commits](abravalheri/validate-pyproject@v0.0.1...v0.20.2)

---
updated-dependencies:
- dependency-name: validate-pyproject
  dependency-type: direct:production
...

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

pyproject.toml

@@ -104,7 +104,7 @@ optional-dependencies.develop = [
   "pyproject-fmt<2.3",
   "ruff<0.7",
   "sphinx-autobuild==2021.3.14", # Newer versions stopped "watching" appropriately?
-  "validate-pyproject<0.20",
+  "validate-pyproject<0.21",
 ]
 optional-dependencies.docs = [
   "furo",