web: Start upgrade to React Router v6
Created by: umpox
Description
React Router V6 is out, and they have just published a migration plan.
This PR is following the "Setting up" stage of the migration plan, which will allow us to start using the V6 router and incrementally upgrade the rest of our app.
Migration will be tackled in this issue: https://github.com/sourcegraph/sourcegraph/issues/33834
Test plan
- Run the app locally
- Integration tests
App preview:
Check out the client app preview documentation to learn more.