Bump version to 7.20.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.19.0...69b7e649188d9bc94fe759b8efdbb27ed790538b

• Release notes:

### Other changes (CI, backstage)

- !833 Add Grafana graph to test cases.

• 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.