Bump onnx from 1.18.0 to 1.21.0

Bumps [onnx](https://github.com/onnx/onnx) from 1.18.0 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@v1.18.0...v1.21.0)

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

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Author: dependabot[bot]