Update validate-pyproject requirement from <0.19 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] <support@github.com>

pyproject.toml

@@ -66,7 +66,7 @@ develop = [
   "poethepoet<1",
   "pyproject-fmt<2.2",
   "ruff<0.6",
-  "validate-pyproject<0.19",
+  "validate-pyproject<0.21",
 ]
 release = [
   "build<2",