build(deps-dev): bump eslint-plugin-mozilla in /frontend

Bumps [eslint-plugin-mozilla](http://firefox-source-docs.mozilla.org/tools/lint/linters/eslint-plugin-mozilla.html) from 4.0.0 to 4.2.1.

---
updated-dependencies:
- dependency-name: eslint-plugin-mozilla
  dependency-version: 4.2.1
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

frontend/package-lock.json

@@ -34,7 +34,7 @@
     "eslint-webpack-plugin": "^4.2.0",
     "eslint-plugin-fetch-options": "0.0.5",
     "eslint-plugin-html": "^8.1.3",
-    "eslint-plugin-mozilla": "^4.0.0",
+    "eslint-plugin-mozilla": "^4.2.1",
     "eslint-plugin-no-unsanitized": "^4.0.1",
     "eslint-plugin-prettier": "^5.4.0",
     "file-loader": "^6.0.0",