Bump mongoose from 5.13.8 to 5.13.15

Bumps [mongoose](https://github.com/Automattic/mongoose) from 5.13.8 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@5.13.8...5.13.15)

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

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Author: dependabot[bot]