We should upgrade to ESLint 9 now that it's officially released. The changelog, including the breaking changes that should be audited, can be found at https://github.com/eslint/eslint/releases/tag/v9.0.0. Note that this upgrade is currently blocked on the ESLint plugins that we use indicating support for ESLint 9 in their package.json ranges first, so this is mostly a tracking issue at the moment.