Bump mongoose from 5.7.1 to 5.7.5 in /app-server

Bumps [mongoose](https://github.com/Automattic/mongoose) from 5.7.1 to 5.7.5.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/History.md)
- [Commits](Automattic/mongoose@5.7.1...5.7.5)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

app-server/package-lock.json

@@ -24,7 +24,7 @@
   "homepage": "https://github.com/ucfcs/spring2019-group23#readme",
   "dependencies": {
     "express": "^4.17.1",
-    "mongoose": "^5.7.1",
+    "mongoose": "^5.7.5",
     "multer": "^1.4.2",
     "path": "^0.12.7",
     "ws": "^7.2.0"