Update docs on min node.js version

Author: sebastianquek

CONTRIBUTING.md

@@ -5,7 +5,7 @@
 `serpapi-javascript` is developed using Deno. Refer to
 https://deno.land/manual/getting_started/installation to install Deno.
 
-Verify that you're running at least Deno v1.28.3.
+Ensure you're running at least Deno v1.28.3.
 
 ```bash
 deno --version

README.md

@@ -17,6 +17,8 @@ more.
 
 ### Node.js
 
+Ensure you're running at least Node.js v16.14.
+
 ```bash
 npm install serpapi
 ```

docs/migrating_from_google_search_results_nodejs.md

@@ -51,6 +51,8 @@ migrate over to the `serpapi` npm package.
 - The `buildUrl`, `execute` and `search` methods are removed. Use `json` and
   `html` functions instead.
 - The `SerpApiSearch` class is removed as a public class.
+- Dropped support for Node.js 16.13 and below. This module supports Node.js
+  16.14 and above.
 
 ## Fixed