Upgrades the `lib` target in `packages/test-utils/tsconfig.json` from `ES2021` to `ES2023`.
This resolves 20 type errors related to `ErrorOptions`, `cause` support in `Error`, and modern `Intl` features (like `Intl.Segmenter`) that were causing build failures in the `test-utils` package and its dependents.
This change aligns `test-utils` with other packages in the monorepo (like `core` and `cli`) that already use `ES2023`.
cc @google-gemini/gemini-cli-maintainers