Update modoboa requirement from >=1.15.0 to >=2.9.0 (#22)

Updates the requirements on [modoboa](https://github.com/modoboa/modoboa) to permit the latest version.
- [Release notes](https://github.com/modoboa/modoboa/releases)
- [Changelog](https://github.com/modoboa/modoboa/blob/master/CHANGELOG.md)
- [Commits](modoboa/modoboa@1.15.0...2.9.0)

---
updated-dependencies:
- dependency-name: modoboa
  dependency-version: 2.9.0
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -1 +1 @@
-modoboa>=1.15.0
+modoboa>=2.9.0