Update version to 8.4.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.3.2...401da1f816952da95fc6993f70ca36a10337658f

• Release notes:

### Other changes (CI, backstage)

- !1026 Mount knapsack reports for all spec runs

• 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.