Update react-router monorepo to v6

[bobsilverberg]

This is one of the remaining dependencies we have not yet updated due to changes needed. Getting to v6 is a multi-step process:

1. Update from v4 to v5. This should be a simple update.
2. Make some changes to our code with v5 to prepare for the update to v6.
3. Install the v5 to v6 compatibility package, which will allow some of our code can continue to use v5 while we migrate others, incrementally to v6.
4. Once all code has been migrated to work with v6, update to v6.

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