build!: update library to use Node 12 (#325)

* feat!: Update library to use Node 12

* 🦉 Updates from OwlBot post-processor

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

Author: alexander-fenster

cloudbuild/package.json

@@ -9,7 +9,7 @@
   "author": "Google Inc.",
   "repository": "googleapis/nodejs-cloudbuild",
   "engines": {
-    "node": ">=10"
+    "node": ">=12.0.0"
   },
   "scripts": {
     "test": "c8 mocha system-test --timeout=800000"