Switch to React Router once v6 is released

[trivikr]

Is your feature request related to a problem? Please describe.
Workshop repo uses @reach/router which was put on path of deprecation in favor of React Router
Post from May 17th, 2019 https://reacttraining.com/blog/reach-react-router-future/

Describe the solution you'd like
Switch to Reach Router once v6 stable version is released.
As of June 2020, it's in beta https://github.com/ReactTraining/react-router/releases/tag/v6.0.0-beta.0

Describe alternatives you've considered
Stick with @reach/router, as it continues to get bugfixes.

[trivikr]

Will be followed in the notes-app written using v3 aws-samples/aws-sdk-js-notes-app#26