Restore QGAN model

[alecandido]

Because of incompatibilities between Keras 3 and Python 3.12, we had to temporarily drop the QGAN model, which was making use of tf.keras.

The model can be restored either in Qibo or QiboML (whenever it will be ready), but the non-trivial problem with Keras 3 means that it has to be rewritten without making use of tf.keras (unless Keras team will change its mind).

keras-team/keras#13127
keras-team/keras#1956
tensorflow/tensorflow#65306

The model is of course available in Git, and the latest release supporting it will be v0.2.7.
https://github.com/qiboteam/qibo/blob/v0.2.7/src/qibo/models/qgan.py