Admin UI: cleaned up react-router usage to match latest recommendations

[Vultraz]

As of react-router 5.1 it's recommended to use children + hooks instead of render or component. 5.1 also included a fix to children so they no longer render when the route doesn't match.

These changes should make it easier for me to upgrade to v6 once it comes out.

• Uses children for routes instead of render. I decided to stick with prop form since this is going to be renamed element in router v6 with children reserved for nested routes.
• Moves the no-lists-defined page into its own file to be consistent with the other error/info pages.
• Utilized useParams for route params

Minor code formatting:

• HomePage: moved the DocTitle down into <main> and dropped the enclosing Fragment
• ListNotFoundPage: simplified string construction

[changeset-bot]

🦋 Changeset is good to go

Latest commit: 1665f98

We got this.

This PR includes changesets to release 1 package

Name	Type
@keystonejs/app-admin-ui	Patch

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[timleslie]

👍

[Vultraz]

Blah, why does this "Async callback was not invoked within the 5000ms" test failure keep happening :(

[timleslie]

Yeah, I dunno, but it's a right pain in the butt :-( If you or someone else has some time to get to the bottom of it that'd be great.