Bump mongoose from 5.13.5 to 5.13.15

Bumps [mongoose](https://github.com/Automattic/mongoose) from 5.13.5 to 5.13.15.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@5.13.5...5.13.15)

---
updated-dependencies:
- dependency-name: mongoose
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -31,7 +31,7 @@
     "express-validator": "^6.12.1",
     "gravatar": "^1.8.1",
     "jsonwebtoken": "^8.5.1",
-    "mongoose": "^5.13.5",
+    "mongoose": "^5.13.15",
     "multer": "^1.4.2",
     "normalize": "^0.3.1",
     "path": "^0.12.7",