Bump version to 6.20.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.19.0...ed968d4365cdb5202d26f6e3f598a6afba67b9be

• Release notes:

### Other changes (CI, backstage)

- !635 Add GEO_FAILOVER variable to set GitLab address for failover tests
- !636 Set QA_FAILURES_MAX_DIFF_RATIO to 0.15 in the CI config

• 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.