build(deps-dev): bump workbox-webpack-plugin in /build/frontend-legacy

Bumps [workbox-webpack-plugin](https://github.com/googlechrome/workbox) from 7.3.0 to 7.4.0.
- [Release notes](https://github.com/googlechrome/workbox/releases)
- [Commits](GoogleChrome/workbox@v7.3.0...v7.4.0)

---
updated-dependencies:
- dependency-name: workbox-webpack-plugin
  dependency-version: 7.4.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]