Bump version to 10.4.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v10.3.0...9a74661fcfef1b55d35a72229d5a76e0d7a43c8a

• Release notes:

### New scenarios and scenario updates

- !1167 Switch configuration for single db
- !1169 Update Gitaly/Praefect to use new format config

### Fixes

- !1166 Fix Selenoid grid setup

• 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.

• Checklist after gitlab-qa version has been released by the post merge pipeline:

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