fix: package.json & yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-WEBPACK-3358798
- https://snyk.io/vuln/SNYK-JS-WEBPACK-7840298
- https://snyk.io/vuln/SNYK-JS-NANOID-2332193
- https://snyk.io/vuln/SNYK-JS-POSTCSS-5926692
- https://snyk.io/vuln/SNYK-JS-TERSER-2806366

package.json

@@ -15,7 +15,7 @@
     "lodash": "^4.17.21",
     "minimist": "1.2.3",
     "popper.js": "1.14.7",
-    "purgecss-webpack-plugin": "^4.0.3",
+    "purgecss-webpack-plugin": "^7.0.2",
     "rails-erb-loader": "^5.5.2",
     "resolve-url-loader": "^4.0.0",
     "sass": "^1.25.0",