Bump nodemailer from 6.9.1 to 6.9.9 in /src/backend

Bumps [nodemailer](https://github.com/nodemailer/nodemailer) from 6.9.1 to 6.9.9.
- [Release notes](https://github.com/nodemailer/nodemailer/releases)
- [Changelog](https://github.com/nodemailer/nodemailer/blob/master/CHANGELOG.md)
- [Commits](https://github.com/nodemailer/nodemailer/compare/v6.9.1...v6.9.9

)

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

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