Bump version to 9.0.0

Diff

v8.0.0...ba2ee5f6 (basically just !119 (merged))

Checklist

• Diff link is up-to-date.
• Check the release notes: https://gitlab.com/api/v4/projects/4176070/repository/changelog?version=9.0.0
• Based on the diff and the release notes, version.rb is updated, according to SemVer.