changes the type of next to be a Promise<any> in koa-convert (#28826)

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The 'Koa.Middleware' format is `(ctx: Context, next: () => Promise<any>)` so I've updated the accepted type here to match up.

types/koa-convert/index.d.ts

@@ -7,7 +7,7 @@
 import { Context, Middleware } from "koa";
 
 declare function convert(
-    mw: (context: Context, next: () => void) => Generator
+    mw: (context: Context, next: () => Promise<any>) => Generator
 ): Middleware;
 
 export = convert;