update node engines to match node versions we are testing against

Author: kiwiupover

package.json

@@ -15,7 +15,7 @@
     "url": "git+https://github.com/ember-fastboot/fastboot.git"
   },
   "engines": {
-    "node": "^6.14.0 || ^8.10.0 || >=9.10.0"
+    "node": "^8.10.0 || 10.* || >=12"
   },
   "keywords": [
     "ember",