License adapt for API plugin

[jonesbusy]

See https://groups.google.com/g/jenkinsci-dev/c/POxcfeUTRzo

Testing done

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[basil]

BTW I just read this article about how CC0 is preferable to Unlicense for public domain dedications.