Bump onnx from 1.19.0 to 1.21.0

Bumps [onnx](https://github.com/onnx/onnx) from 1.19.0 to 1.21.0.
- [Release notes](https://github.com/onnx/onnx/releases)
- [Changelog](https://github.com/onnx/onnx/blob/main/docs/Changelog-ml.md)
- [Commits](onnx/onnx@v1.19.0...v1.21.0)

---
updated-dependencies:
- dependency-name: onnx
  dependency-version: 1.21.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

pyproject.toml

@@ -72,7 +72,7 @@ dev = [
 ]
 apps = [
   # for apps/onnx -- matches the vcpkg version
-  "onnx==1.19.0; platform_machine != 'armv8l' and platform_machine != 'armv7l'",
+  "onnx==1.21.0; platform_machine != 'armv8l' and platform_machine != 'armv7l'",
   # onnx 1.19.0 gencode requires protobuf 7.x runtime
   "protobuf>=7; platform_machine != 'armv8l' and platform_machine != 'armv7l'",
   # unspecified onnx dependency