Add the support for LightGBM 4.6.0

[c-bata]

Expected behavior

LightGBM 4.6.0 was released two days ago, causing the workflow to fail as shown below.

• https://github.com/optuna/optuna-examples/actions/runs/13356151962
• https://github.com/optuna/optuna-integration/actions/runs/13359662201

I opened a PR to add the version constraint, lightgbm<=4.5.0, as a hotfix (optuna/optuna-examples#302). This issue suggests fixing the above error and remove the version constraint on optuna-examples and optuna-integration.

Environment

Please check out the CI logs.

Error messages, stack traces, or logs

Please check out the CI logs.

* https://github.com/optuna/optuna-examples/actions/runs/13356151962
* https://github.com/optuna/optuna-integration/actions/runs/13359662201

Steps to reproduce

Please run unittests in optuna-integration with the latest LightGBM version.

Additional context (optional)

No response