Use `suggest_float` instead of `suggest_loguniform`.

[toshihikoyanase]

Thank you for your great project! I just found a point I could contribute although I'm not sure if this repository receives pull requests. Please feel free to close the PR if you don't need it.

Motivation

• Trial.suggest_loguniform will be deprecated according to https://github.com/optuna/optuna/issues/2939.
• It recommends Trial.suggest_float(..., log=True) instead of it.

Changes

• This PR simply replaces Trial.suggest_loguniform with Trial.suggest_float.