fix: downgrade koa to v2.16.0

Author: titanism

package.json

@@ -23,7 +23,7 @@
     "cabin": "^14.0.0",
     "express-request-id": "1.4.1",
     "kcors": "^2.2.2",
-    "koa": "^3.0.0",
+    "koa": "2.16.0",
     "koa-404-handler": "^0.1.0",
     "koa-basic-auth": "^4.0.0",
     "koa-better-error-handler": "^11.0.4",