Bump VERSION to 4.20.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v.4.19.3...921642730075809be60472824d29ae4a668689d8

• Release notes:

### New scenarios and scenario updates

- !347 Add Test::Instance::RepositoryStorage scenario

• 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.