Update koa-generator deprecated dependency.
One of the dependencies (mkdirp@0.5.1) of koa-generator is deprecated.

```javascript
npm warn deprecated mkdirp@0.5.1: Legacy versions of mkdirp are no longer supported. Please update to mkdirp 1.x. (Note that the API surface has changed to use Promises in 1.x.)
```
![Screenshot 2024-06-06 at 12 28 38 PM](https://github.com/i5ting/koa-generator/assets/12603244/9408d792-9bec-42b7-8609-7d5583018c88)
