build(deps): bump onnx from 1.18 to 1.21.0rc1

Bumps [onnx](https://github.com/onnx/onnx) from 1.18 to 1.21.0rc1.
- [Release notes](https://github.com/onnx/onnx/releases)
- [Changelog](https://github.com/onnx/onnx/blob/main/docs/Changelog-ml.md)
- [Commits](https://github.com/onnx/onnx/commits)

---
updated-dependencies:
- dependency-name: onnx
  dependency-version: 1.21.0rc1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

pyproject.toml

@@ -30,7 +30,7 @@ dependencies = [
   "jsonschema>=4.23",
   "ml-dtypes>=0.5.3",
   "numpy>=2.2,<2.3",
-  "onnx==1.18",
+  "onnx==1.21.0rc1",
   "onnx-ir>=0.1",
   "onnxruntime-windowsml>=1.23.2,<1.24.0",
   "onnxscript>=0.2",