Bump version to 6.0.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.17.1...cbc50c56e20ca4e1c36921ce381e58137d8cd227

• Release notes:

### New scenarios and scenario updates

- !500 Rename Gitaly HA to Gitaly Cluster (**breaking change**)

### Other changes (CI, backstage)

- !509 Fix reporting command
- !503 Report test results from JSON files

• 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.