Bump version to 1.28.0

Diff

v1.27.0...253ee0d1

Checklist

• Diff link is up-to-date.
• Check the release notes: https://gitlab.com/api/v4/projects/3430480/repository/changelog?version=1.28.0
  • I completely missed to add a changelog entry for !270 (merged) 😥 I'll add it to the release note manually once this is released.
• Based on the diff and the release notes, version.rb is updated, according to SemVer.