Bump the opset version in README.md

[Lutra-Fs]

Is your feature request related to a problem? Please describe.
Seems like the opset in README.md is 16. Any plan to update that to include further changes in onnx?

Describe the solution you'd like
upgrade the opset version to 20?

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[pixelspark]

We are not very strict at implementing a specific version of the opset currently (in cases where there are differences, the code does sometimes check and behave differently for different versions, but generally it doesn't check when e.g. newer versions of an op are backward compatible).

We could reference the most recent opset from README of course, and we should mention the version supported.