Update dependency sequelize to v6.21.6

Author: renovate[bot]

package-lock.json

@@ -31,7 +31,7 @@
     "pug": "3.0.2",
     "qs": "6.11.0",
     "respond.js": "1.4.2",
-    "sequelize": "6.21.4",
+    "sequelize": "6.21.6",
     "uuid": "8.3.2"
   },
   "devDependencies": {