Bump github.com/xhit/go-simple-mail/v2 from 2.13.0 to 2.15.0

Bumps [github.com/xhit/go-simple-mail/v2](https://github.com/xhit/go-simple-mail) from 2.13.0 to 2.15.0.
- [Release notes](https://github.com/xhit/go-simple-mail/releases)
- [Commits](https://github.com/xhit/go-simple-mail/compare/v2.13.0...v2.15.0)

---
updated-dependencies:
- dependency-name: github.com/xhit/go-simple-mail/v2
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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