Bump VERSION to 2.6.0

• Diff: v2.5.0...6e97ec5f

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, version.rb is updated, according to SemVer.

• Checklist after merging:

  • Update the release notes for the newly created tag.
  • Check that automatic release notes (generated following the same process as https://docs.gitlab.com/ee/development/changelog.html) are correct.