Bump eslint-plugin-vue from 7.20.0 to 9.1.0 in /frontend

Bumps [eslint-plugin-vue](https://github.com/vuejs/eslint-plugin-vue) from 7.20.0 to 9.1.0.
- [Release notes](https://github.com/vuejs/eslint-plugin-vue/releases)
- [Commits](https://github.com/vuejs/eslint-plugin-vue/compare/v7.20.0...v9.1.0)

---
updated-dependencies:
- dependency-name: eslint-plugin-vue
  dependency-type: direct:development
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

frontend/package.json

@@ -28,7 +28,7 @@
     "@vue/cli-service": "~4.5.15",
     "babel-eslint": "^10.1.0",
     "eslint": "^6.7.2",
-    "eslint-plugin-vue": "^7.20.0",
+    "eslint-plugin-vue": "^9.1.0",
     "vue-template-compiler": "^2.6.14",
     "workbox-webpack-plugin": "^6.5.3"
   },