Bump version to 7.3.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.2.1...18cbc4457056119b5fab6a5f1ba373344dc71446

• Release notes:

### Doc changes

- !704 Pass generate allure report environment variable.

### Other changes (CI, backstage)

- !704 Pass generate allure report environment variable.

• 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.