Bump npm from 6.4.1 to 6.13.4 in /example/frontend/src/main/frontend

Bumps [npm](https://github.com/npm/cli) from 6.4.1 to 6.13.4.
- [Release notes](https://github.com/npm/cli/releases)
- [Changelog](https://github.com/npm/cli/blob/latest/CHANGELOG.md)
- [Commits](npm/cli@v6.4.1...v6.13.4)

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Author: dependabot[bot]

example/frontend/src/main/frontend/package.json

@@ -76,7 +76,7 @@
     "babel-polyfill": "^6.26.0",
     "bootstrap": "^4.3.1",
     "html-validator-cli": "^4.0.0",
-    "npm": "^6.4.1",
+    "npm": "^6.13.4",
     "postcss-cli": "^5.0.0",
     "serve": "^6.5.5",
     "slick-carousel": "^1.8.1",