Bump version to 6.20.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.20.0...fab5ada51c2b673dd0fb6630ca46eae3096dfa15

• Release notes:

### Fixes

- !638 Create a note in session report only if QA issue is defined.

• 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:

  • Update the release notes for the newly created tag.