[eprh] Update plugin config to be compatible with flat and legacy

This has been incredibly frustrating as [ESLint's own docs](https://eslint.org/docs/latest/extend/plugins#backwards-compatibility-for-legacy-configs) are clearly wrong (see #34679).

This PR uses [eslint-plugin-react's setup](https://github.com/jsx-eslint/eslint-plugin-react/blob/master/index.js) as a reference, where the presets are assigned  to `configs.flat` (not documented by eslint).