fix(deps): update dependency gatsby-plugin-purgecss to v4 (decaporg#237)

Author: renovate[bot]

package-lock.json

@@ -9,7 +9,7 @@
     "gatsby-image": "^2.0.23",
     "gatsby-plugin-netlify": "^2.0.6",
     "gatsby-plugin-netlify-cms": "^4.0.0",
-    "gatsby-plugin-purgecss": "^3.1.0",
+    "gatsby-plugin-purgecss": "^4.0.0",
     "gatsby-plugin-react-helmet": "^3.0.4",
     "gatsby-plugin-sass": "^2.0.7",
     "gatsby-plugin-sharp": "^2.0.15",