Update Cython dependency

[alexander-pv]

Proposed changes

Hi, I noticed that recent Cython builds break causalml package building process.
I added an upper bound for cython version to make everything work as expected.
I also updated Github action versions.

Turned out that an old PR completely rewrote the latest causal_trees_with_synthetic_data.ipynb.
So I returned back conterfactual outcomes example in causal trees notebook.

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[jeongyoonlee]

HI @alexander-pv. Can you check if the fix proposed in #642 addresses the build failure with latest Cython? If so, we don't need the Cython version restriction.

[jeongyoonlee]

Just saw your comment in #534. Please discard my comment above. It looks good to me. Thanks!