Update "react-router-dom" and "react-toastify" dependencies

These are major updates and code changes are necessary (at least for react router).

https://reactrouter.com/docs/en/v6/upgrading/v5#upgrade-all-switch-elements-to-routes

https://github.com/fkhadra/react-toastify/releases/tag/v8.0.0