test(nodejs): remove 15 add 16 (#1322) (#918)

Source-Link: googleapis/synthtool@6981da4
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:3563b6b264989c4f5aa31a3682e4df36c95756cfef275d3201508947cbfc511e

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

packages/google-cloud-dialogflow/README.md

@@ -6,7 +6,6 @@
 
 [![release level](https://img.shields.io/badge/release%20level-general%20availability%20%28GA%29-brightgreen.svg?style=flat)](https://cloud.google.com/terms/launch-stages)
 [![npm version](https://img.shields.io/npm/v/@google-cloud/dialogflow.svg)](https://www.npmjs.org/package/@google-cloud/dialogflow)
-[![codecov](https://img.shields.io/codecov/c/github/googleapis/nodejs-dialogflow/main.svg?style=flat)](https://codecov.io/gh/googleapis/nodejs-dialogflow)