Preparation for v3.0.0

[fhammerschmidt]

What's missing for the release of v3.0.0?

• [x] Test rc.1 in some of our projects
• [x] Migration Guide
• [x] Update the PPX package as well
• [x] Check all docs one last time for errors, inconsistencies, etc.
• [x] Update changelog

Nice to have, but not required: the remaining open issues.