Set Node v12 to available nodesource version

- seems v12.10.0 is the latest available version for bionic via nodesource ppa
- want to support a version that can be installed on our recommended stack

package.json

@@ -22,7 +22,7 @@
     "lint:js": "eslint ."
   },
   "engines": {
-    "node": "^8.16.0 || ^10.13.0 || ^12.11.1"
+    "node": "^8.16.0 || ^10.13.0 || ^12.10.0"
   },
   "devDependencies": {
     "@ember/jquery": "1.1.0",