vercel: explicitly set version to v12

v12 is currently the default now,
https://vercel.com/docs/runtimes#official-runtimes/node-js/node-js-version,
but we'd rather be explicit about the version we use and ensure that it
matches local development.

website/package.json

@@ -3,6 +3,9 @@
   "description": "Description of your website",
   "version": "0.0.1",
   "author": "HashiCorp",
+  "engines": {
+    "node": "12.x"
+  },
   "dependencies": {
     "@hashicorp/nextjs-scripts": "^13.2.0",
     "@hashicorp/react-alert-banner": "4.2.0",