Bump version to 6.21.3

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.21.2...eb286909bcc7bf1f2fd33dc4ddfbc2220893587a

• Release notes:

### Fixes

- !651 Remove link to testcase when test passed

• 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.