Bump webpack-dev-middleware in /integration/app/test-demo

Bumps [webpack-dev-middleware](https://github.com/webpack/webpack-dev-middleware) from 5.3.1 to 5.3.4.
- [Release notes](https://github.com/webpack/webpack-dev-middleware/releases)
- [Changelog](https://github.com/webpack/webpack-dev-middleware/blob/v5.3.4/CHANGELOG.md)
- [Commits](webpack/webpack-dev-middleware@v5.3.1...v5.3.4)

---
updated-dependencies:
- dependency-name: webpack-dev-middleware
  dependency-type: indirect
...

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