Bump version to 6.13.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.12.0...84167a204b5b3a8d63153adf4b3fe68da55f76ef

• Release notes:

### New scenarios and scenario updates

- !594 Resolve "Failure issues were not grouped in the test session report"
- !595 Resolve "Add ~found:* label on the test session report based on environment"
- !597 Resolve "Test session report should handle its own failed jobs"

### Fixes

- !593 Work around a bug due to 'Gitlab::ObjectifiedHash#hash' being overriden

• 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.