Fixed rjsf-team#3169 by merging `extraErrors` in while preventing duplicates
- Updated `@rjsf/utils` to make the `mergeObjects()` function support a `preventDuplicates` mode when concatenating arrays
  - Updated the tests to verify this new feature
- Updated `@rjsf/core` to use the `preventDuplicates` mode when merging `extraErrors` when not live validating
- Updated `validation.md` to remove extraneous leading space to fix rjsf-team#3282
- Updated `utility-functions.md` to document the new mode
- Updated `index.md` to provide the proper `core.cjs.production.min.js` package name for `unpkg.com` fixing rjsf-team#3262
- Updated the `CHANGELOG.md` accordingly