Migration to eslint v9

[DanWebb]

Eslint v9 forces a migration to a new "flat" config file https://eslint.org/docs/latest/use/configure/migration-guide

Projects won't be able to extend this eslint config in v9 until we move to the flat config format.

As of writing, attempting to install eslint v9 gives strict peer dependency errors for

• eslint-plugin-react (ESLint v9 contains breaking API changes jsx-eslint/eslint-plugin-react#3699)
• eslint-plugin-import (Support eslint v9 import-js/eslint-plugin-import#2948)

There is compatibility utilities for the other plugins that haven't moved to a flat format yet like You-Dont-Need-Lodash-Underscore but reading into it my understanding is the strict peer dependency for these two is intentional because there's breaking API changes.