Upgrade `react-router-dom` dependency in the dashboard

We're on `5.3.0` and would like to upgrade to `v6`, but that's a breaking change that requires manual intervention in our routes handling.

The React-router project has promised a compatibility layer to come up at some point that would avoid having to change things too much, but I feel that's a second-best solution and would prefer to have things written as per the best practice.

For anyone wanting to tackling this, we're glad to provide guidance on the overall setup of the dashboard web app in linkerd (also note that BUILD.md provides lots of details).

Start with BUILD.md for the dashboard web app setup and inspect the dashboard's current routes handling around the react-router-dom 5.3.0 dependency. Identify the manual changes required for the v6 migration, then verify that the dashboard routes work with the upgraded dependency and follow React Router v6 practices.

Written by the indexing model from the issue text.