Bump mongoose from 5.4.17 to 5.7.5 in /server

Bumps [mongoose](https://github.com/Automattic/mongoose) from 5.4.17 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@5.4.17...5.7.5)

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

server/package.json

@@ -33,7 +33,7 @@
     "express-session": "^1.15.6",
     "jsonwebtoken": "^8.5.0",
     "lodash": "^4.17.11",
-    "mongoose": "^5.4.17",
+    "mongoose": "^5.7.5",
     "morgan": "^1.9.1",
     "multer": "^1.4.1",
     "passport": "^0.4.0",