Migration guide for Eclipse 4.39

Migration guide for Eclipse 4.39

Done via

- https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/pull/3670
- https://github.com/eclipse-platform/www.eclipse.org-eclipse/pull/496
- https://github.com/eclipse-platform/eclipse.platform/pull/2467
- https://github.com/eclipse-jdt/eclipse.jdt/pull/155