Update version to 8.4.2

Diff

https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.4.1...d00d08ec8ce3938579adbb1e9e36df19fe4ec2fb

Release notes

Fixes

• Fixes version command (!1035 - merged)
• Use the working v2 registry API (!1037 - 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.