Upgrade react-router

[jasonaowen]

react-router recently released version 7.0.0. Upgrading from v6 will take some work.

Dependabot PR #899 attempted a major version bump, but there is more than that to be done:

• upgrade from Node 18 to Node 20
• rename react-router-dom to react-router (although the react-router-dom package is currently provided as a convenience, it won't be there forever, and if we need to make other breaking changes then we might as well future-proof this as part of that effort)
• review the v6 feature flags and make sure that we can use the new defaults
• figure out what the linter is now complaining about