Clarify Missing <Routes> in UPGRADE_GUIDE

[appsforartists]

In the Upgrade Guide, you mention that <Routes> is gone, but don't provide an example of a new route tree. This could be confusing to users who weren't following new-api.

I recommend adding an example of the new route tree, and perhaps renaming routes to routeTree or routeRoot in the guide.

[ryanflorence]

thanks, that was probably kinda confusing