build!: update library to use Node 12 (#371)

* build!: Update library to use Node 12
Co-authored-by: Summer Ji <summerji@google.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: 3 people

secret-manager/package.json

@@ -8,7 +8,7 @@
   "author": "Google LLC",
   "repository": "googleapis/nodejs-secret-manager",
   "engines": {
-    "node": ">=10"
+    "node": ">=12.0.0"
   },
   "scripts": {
     "test": "c8 mocha --recursive test/ --timeout=800000"
@@ -22,4 +22,4 @@
     "mocha": "^8.0.0",
     "uuid": "^8.0.0"
   }
-}
+}