Bump version to 5.8.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.8.0...8dfdc1238868040fe4c93c236541460c941525ba

• Release notes:

### Fixes

- !445 Put back GITLAB_QA_ACCESS_TOKEN.

• 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.