build(node): upgrade to Node.js v18

Author: ivangabriele

docs/_media/demo.gif

@@ -6,7 +6,7 @@
   "private": true,
   "type": "module",
   "engines": {
-    "node": "16",
+    "node": "18",
     "npm": "8",
     "pnpm": "7"
   },

package.json

@@ -7,10 +7,11 @@
   "repository": "https://github.com/betagouv/tell-me",
   "success_url": "/",
   "website": "https://www.tell-me.org",
+  "stack": "scalingo-20",
   "env": {
     "BUILDPACK_URL": {
       "description": "Custom buildpack to support pnpm (don't change the default value).",
-      "value": "https://github.com/unfold/heroku-buildpack-pnpm"
+      "value": "https://github.com/unfold/heroku-buildpack-pnpm#main"
     },
     "NODE_ENV": {
       "description": "Application environment (don't change the default value).",