Why doesn't the latest version of pyTorch support ONNX opset version: 20 ???

dummy_input = torch.randn(1, 1, 28, 28)
torch.onnx.export(model, dummy_input, "stn_classifier.onnx", verbose=True, opset_version=20)

vesion:
torch 2.2.2+cu118
torchsummary 1.5.1
torchvision 0.17.2+cu118