Bump VERSION to 6.8.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.7.0...8112657

• Release notes:

### New scenarios and scenario updates

- !526 Generate test session report.
- !546 Relate existing failure issues upon test failures or create new ones.

### Fixes

- !555 Add `GitlabIssueDryClient` to autoload.
- !556 Convert the set into an array for the API.
- !557 Ensure we don't call `#web_url` when issue wasn't created.
- !560 Recognise staging-orchestrated pipelines.

### Other changes (CI, backstage)

- !558 Add `when` for generate_test_session rules.

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Create a tag for the new release version.