Update dependency koa to v2.12.0

Author: renovate-bot

package.json

@@ -24,7 +24,7 @@
     "@next/bundle-analyzer": "9.3.4",
     "babel-plugin-lodash": "3.3.4",
     "gatsby": "2.20.10",
-    "koa": "2.11.0",
+    "koa": "2.12.0",
     "lodash": "4.17.15",
     "next": "9.3.4",
     "next-build-id": "3.0.0",