Bump nodemailer and @types/nodemailer in /backend

Bumps [nodemailer](https://github.com/nodemailer/nodemailer) and [@types/nodemailer](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/nodemailer). These dependencies needed to be updated together.

Updates `nodemailer` from 6.9.14 to 6.9.15
- [Release notes](https://github.com/nodemailer/nodemailer/releases)
- [Changelog](https://github.com/nodemailer/nodemailer/blob/master/CHANGELOG.md)
- [Commits](nodemailer/nodemailer@v6.9.14...v6.9.15)

Updates `@types/nodemailer` from 6.4.15 to 6.4.16
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/nodemailer)

---
updated-dependencies:
- dependency-name: nodemailer
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: "@types/nodemailer"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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