fix: package.json & package-lock.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-MPATH-1577289

Author: snyk-bot

package-lock.json

@@ -24,7 +24,7 @@
     "express-validator": "^6.12.1",
     "gravatar": "^1.8.2",
     "jsonwebtoken": "^8.5.1",
-    "mongoose": "^6.0.2",
+    "mongoose": "^6.0.4",
     "request": "^2.88.2"
   },
   "devDependencies": {