Bump version to 12.4.0

Diff

https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v12.3.0...b6926d5753ca64c7e588413f6577c09d9e463b7a

Pre-merge checklist

• Diff link is up-to-date.
• Check the release notes: https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab-qa/repository/changelog?version=v12.4.0
• Based on the diff and the release notes, version.rb is updated, according to SemVer.

Post-merge checklist

• In the pipeline-common project, update the GITLAB_QA_VERSION and create a release.
• In the GitLab project, update the ref for pipeline-common in .gitlab/ci/qa-common/main.gitlab-ci.yml.
• Unless already done by renovate-gitlab-bot, or if you need it sooner, in the GitLab project, update the gitlab-qa gem version in qa/Gemfile and qa/Gemfile.lock (for an example, see: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/117054).