upgrade eslint to v9

[ren-yamanashi]

Overview

The current version of eslint is v8, but it seems to have already passed EOL.
see: https://eslint.org/blog/2024/09/eslint-v8-eol-version-support/

Therefore, I suggest updating eslint to v9.

Notes

In eslint v9, the default writing method will be flatConfig, so one of the following modifications will be necessary.

1. Rewrite the current config to flatConfig

2. Set the environment variable (ESLINT_USE_FLAT_CONFIG=false) to continue using the config.
   (see: https://eslint.org/docs/latest/use/configure/migration-guide#start-using-flat-config-files)

In v10, environment variables will also be removed, so in the long term, I think it would be better to rewrite it to flatConfig.
see: https://eslint.org/blog/2023/10/flat-config-rollout-plans/#eslintrc-removed-in-eslint-v10.0.0

Acknowledgements

• I may be able to implement this issue