Bump serverless-domain-manager from 7.1.2 to 9.0.0 in /backend

Bumps [serverless-domain-manager](https://github.com/amplify-education/serverless-domain-manager) from 7.1.2 to 9.0.0.
- [Release notes](https://github.com/amplify-education/serverless-domain-manager/releases)
- [Changelog](https://github.com/amplify-education/serverless-domain-manager/blob/main/CHANGELOG.md)
- [Commits](amplify-education/serverless-domain-manager@v7.1.2...v9.0.0)

---
updated-dependencies:
- dependency-name: serverless-domain-manager
  dependency-version: 9.0.0
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]