Release 2.9.0

[stap-m]

1. All issues closed and all pull requests merged that are part of the release milestone.
2. Branch releases-v1.2.3 from dev.
3. Update version information and release date in VERSION file and CITATION.cff.
4. Change unreleased heading in CHANGELOG.md.
5. Create two pull requests.
6. Automated tests are successful.
7. PR reviews.
8. Merge both PRs.
9. Close release milestone.
10. Draft release.
11. Set as pre-release.
12. Press Publish release button.
13. Wait for workflows to complete and check automatically added release assets (build files etc)
14. Once successful: Set as the latest release
15. Create new unreleased section in CHANGELOG.md.
16. Inform @jh-RLI.

Detailled release workflow

The release was done by @areleu @schwarzatoffis and myself

[jh-RLI]

Sorry for the long wait ... i just updated the oeo version on the OEP. @stap-m