build(deps-dev): bump @testing-library/vue in /build/frontend-legacy

Bumps [@testing-library/vue](https://github.com/testing-library/vue-testing-library) from 5.9.0 to 6.6.1.
- [Release notes](https://github.com/testing-library/vue-testing-library/releases)
- [Commits](testing-library/vue-testing-library@v5.9.0...v6.6.1)

---
updated-dependencies:
- dependency-name: "@testing-library/vue"
  dependency-version: 6.6.1
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]