build!: update library to use Node 12 (#73)

* build!: Update library to use Node 12

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* dep: update google-gax

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: sofisl

eventarc/quickstart/package.json

@@ -4,7 +4,7 @@
   "license": "Apache-2.0",
   "author": "Google LLC",
   "engines": {
-    "node": ">=10"
+    "node": ">=12.0.0"
   },
   "files": [
     "*.js"