[Task]: Uncomment ONNX tests once onnx support protobuf 4.x.x

### What needs to happen?

https://github.com/apache/beam/pull/24599 comments onnx code since ONNX doesn't support protobuf 4.x https://github.com/onnx/onnx/issues/4239.

Uncomment the onnx tests once ONNX supports https://github.com/onnx/onnx/issues/4239

### Issue Priority

Priority: 3 (nice-to-have improvement)

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