Bump mongoose from 4.13.20 to 5.7.5 in /server

Bumps [mongoose](https://github.com/Automattic/mongoose) from 4.13.20 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@4.13.20...5.7.5)

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

Author: dependabot[bot]

server/package-lock.json

@@ -15,7 +15,7 @@
     "http-errors": "~1.6.3",
     "jwt-simple": "^0.5.6",
     "mailgun.js": "^2.0.1",
-    "mongoose": "^4.13.20",
+    "mongoose": "^5.7.5",
     "morgan": "~1.9.1",
     "nodemon": "^2.0.2",
     "passport": "^0.4.1",