fix: exercises/udacity-c2-restapi/package.json & exercises/udacity-c2-restapi/package-lock.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-VALIDATOR-13395830

Author: snyk-bot

exercises/udacity-c2-restapi/package-lock.json

@@ -27,7 +27,7 @@
     "jsonwebtoken": "^8.5.1",
     "pg": "^7.11.0",
     "reflect-metadata": "^0.1.13",
-    "sequelize": "^5.15.1",
+    "sequelize": "^6.6.5",
     "sequelize-typescript": "^0.6.11"
   },
   "devDependencies": {