Bump webpack-dev-server from 3.11.2 to 4.7.3

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.11.2 to 4.7.3.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack/webpack-dev-server/compare/v3.11.2...v4.7.3)

---
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>

package.json

@@ -23,7 +23,7 @@
     "karma-chrome-launcher": "~3.1.0",
     "karma-coffee-preprocessor": "~1.0.1",
     "karma-jasmine": "~0.3.8",
-    "webpack-dev-server": "~3"
+    "webpack-dev-server": "~4"
   },
   "license": "AGPL-3.0",
   "dependencies": {