Bump webpack-dev-server from 4.15.2 to 5.0.4 in /spec/example_app (#325)

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 4.15.2 to 5.0.4.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](webpack/webpack-dev-server@v4.15.2...v5.0.4)

---
updated-dependencies:
- dependency-name: webpack-dev-server
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

spec/example_app/package.json

@@ -8,6 +8,6 @@
   },
   "devDependencies": {
     "@webpack-cli/serve": "^2.0.5",
-    "webpack-dev-server": "^4.11.1"
+    "webpack-dev-server": "^5.0.4"
   }
 }