Migrate ESLint to version 9

[phantomjinx]

The config syntax for ESLint has changed exclusively to flat format and as such will need to be correctly migrated. However, it would appear that the eslint-react-plugins are not yet ready for this hence we'd need to wait on them upgrading before doing the same.

In the meantime, we will need to remain at version 8.

[phantomjinx]

ref: https://www.raulmelo.me/en/blog/migration-eslint-to-flat-config