Support Keras 3.0

[nzw0301]

Motivation

The current Optuna's Keras callback has been deprecated since the Keras 2.4 release which was identical to tf.keras. However, Keras 3.0 supports multiple DL frameworks (PyTorch, jax, and TensorFlow) as an independent library again, so the deprecated decorator should be removed if optuna-integration supports Keras.

Description

1. Remove the deprecated decorator in https://github.com/optuna/optuna-integration/blob/main/optuna_integration/keras.py.

Alternatives (optional)

Adding tests of Keras 3.0 with multiple DL frakework backends: pytorch, jax, and tensorflow, seems important to expand users of optuna.Optuna

Additional context (optional)

• Compatibilities between keras 2 and keras 3 are explained in https://keras.io/keras_3/
• Keras 3 document https://keras.io/getting_started/#getting-started-with-keras