Document how to migrate from 3.27 to 4.0

## Deliverables

- [ ] Document breaking changes at https://github.com/assertj/assertj/wiki/AssertJ-4.0-Migration-Guide
- [ ] Define OpenRewrite recipe (something we maintain or just a contribution to [`rewrite-migrate-java`](https://github.com/openrewrite/rewrite-migrate-java)?)