Release v0.3.0

[afrittoli]

• [x] Make sure all issues are resolved: https://github.com/cdevents/spec/milestone/3
• [x] Switch all versions from 0.2.0-draft to 0.2.0 https://github.com/cdevents/spec/pull/131
• [x] Switch all event versions from version-draft to version https://github.com/cdevents/spec/pull/131
• [x] Remove mentions of draft / work-in-progress content https://github.com/cdevents/spec/pull/131
• [x] Update the README to mention 0.2.0 as latest release https://github.com/cdevents/spec/pull/133
• [x] Tag spec and create branch for patch releases
• [x] Produce release notes through GitHub
• [x] Update cdevents.dev as needed https://github.com/cdevents/cdevents.dev/pull/35
• [ ] Prepare a release announcement for v0.3 (see #47 for reference)
• [ ] Announce release on slack and mailing list
• [x] Move main in the spec repo to the new -draft release https://github.com/cdevents/spec/pull/135