Replace old distribution definitions with new ones

[HideakiImamura]

Optuna v3.0 has been released, and it deprecated the old distributions and introduced the new distributions. We should replace the old definitions with new ones.

What we should change.

• Replace the definitions of UniformDistribution, LogUniformDistribution, and DiscreteUniformDistribution with FloatDistribution.
• Replace the definitions of IntDistribution and IntLogUniformDistribution with IntDistribution.

For example, here.