chore(deps): bump onnx-weekly in /requirements/ci

Bumps [onnx-weekly](https://github.com/onnx/onnx) from 1.21.0.dev20251124 to 1.21.0.dev20251215.
- [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-weekly
  dependency-version: 1.21.0.dev20251215
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

requirements/ci/requirements-onnx-weekly.txt

@@ -1 +1 @@
-onnx-weekly==1.21.0.dev20251124
+onnx-weekly==1.21.0.dev20251215