Update version to 7.21.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.20.0...e136d83c460ddde0ccf6556895a512c868db34c1

Release notes:

Other changes (CI, backstage)

• Add date in UTC to beginning of test session re... (!836 - merged)
• Collect orchestrated environment metrics via te... (!837 - merged)
• Remove use of TRIGGERED_USER and TRIGGER_SOURCE (!838 - merged)
• Remove references to outdated staging-orchestrated (!840 - merged)

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.