core: rollback koa types

packages/hydrooj/package.json

@@ -60,7 +60,7 @@
         "@types/adm-zip": "^0.4.34",
         "@types/fs-extra": "^11.0.1",
         "@types/js-yaml": "^4.0.5",
-        "@types/koa": "^2.13.6",
+        "@types/koa": "2.13.5",
         "@types/koa-compress": "^4.0.3",
         "@types/koa-router": "^7.4.4",
         "@types/koa-static-cache": "^5.1.1",