Bump version to 7.4.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v7.3.1...47ef1914f18e5e3500dd900296d340605e966c92

• Release notes:

### New scenarios and scenario updates

- !711 Update default branch to main

• 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.