fix: fixed to mongoose v6

Author: titanism

package.json

@@ -51,7 +51,7 @@
     "ioredis": "^5.3.1",
     "ioredis-mock": "^8.2.7",
     "lint-staged": "13.2.0",
-    "mongoose": "^7.0.3",
+    "mongoose": "6",
     "nyc": "^15.1.0",
     "remark-cli": "^11.0.0",
     "remark-preset-github": "^4.0.4",