Changehange node.js version in readme.md from 12.x to 18.x (#1823)

## Checklist

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Readme.md

@@ -17,7 +17,7 @@
 
 ## Installation
 
-Koa requires __node v12.17.0__ or higher for ES2015 and async function support.
+Koa requires __node v18.0.0__ or higher for ES2015 and async function support.
 
 ```
 $ npm install koa