fix: koa dependencies are outdated

[loriswit]

Description

koa-router, koa-multer and kcors are not the recommended packages anymore. They should be changed to @koa/router, @koa/multer and @koa/cors respectively.

Note that koa-router actually seems to be a mirror of @koa/router, so it might not really be an issue, but koa-multer has been marked as deprecated, and kcors hasn't been updated for 3 years.

Related issues: #428 #435 #534 #604