Fix 4283 by adding support for MUI v6

[heath-freenome]

Reasons for making this change

Fixed #4283 by adding support for MUI v6

• Updated @rjsf/mui to update the dev dependencies for @mui/* to v6 and the peer dependencies to add MUI v6
• Updated the playground to update @mui/* to v6
  • Also changed the dropdown name from material-ui-5 to mui and switched Layout to use the PropsWithChildren for its props
  • Updated the Form.test.tsx to add information to the fake node to allow the tests to work with v6
  • Ran npm run test:update to update the snapshots due to the v6 and emotion upgrades
• Updated the test snapshots for @rjsf/chakra-ui which also uses emotion
• Updated CHANGELOG.md accordingly, bumping to 5.21.0

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[nickgros]

Playground links with material-ui-5 theme encoded in the URL will lose styling since it's now mui, but I think that's ok

[heath-freenome]

Playground links with material-ui-5 theme encoded in the URL will lose styling since it's now mui, but I think that's ok

Hmmm, good point. I wonder if I can keep the selection in the backend, just not show it in the frontend? I updated Playground component to fix that. And tested locally to verify it works