build!: upgrade engines field to >=8.10.0 (#608)

Author: Benjamin E. Coe

handwritten/firestore/package.json

@@ -5,7 +5,7 @@
   "license": "Apache-2.0",
   "author": "Google Inc.",
   "engines": {
-    "node": ">=6.0.0"
+    "node": ">=8.10.0"
   },
   "repository": "googleapis/nodejs-firestore",
   "main": "./build/src/index.js",