Fix RCFR test and example to work with Keras 3

[dmorrill10]

Fixes #1207 by replacing lr with learning_rate and replacing calls to minimize, which is no longer present in tf.keras.optimizers.Adam. The test_rcfr_functions was flaky so I also increased the size of the model to get the test to pass.

With this patch, python open_spiel/python/algorithms/rcfr_test.py should now pass and python open_spiel/python/examples/rcfr_example.py runs without error.

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[lanctot]

Awesome, thanks!

Can you remove this comment:

open_spiel/open_spiel/python/CMakeLists.txt

Line 314 in 7ff9e28

# Broken in Python 3.12. Must port to Keras 3. https://github.com/google-deepmind/open_spiel/issues/1207.

and uncomment the next line, so that the rcfr_test is re-enabled and we can run it on Github Actions CI?

[lanctot]

CI test failing for an unrelated reason, will fix and update master first.

[lanctot]

CI test failing for an unrelated reason, will fix and update master first.

Fixed now. (by #1267, which was merged.)

So if you pull from master and re-enable the test we'll see if it passes on Github Actions.