Bump serverless-domain-manager from 6.2.0 to 7.3.8 in /backend

Bumps [serverless-domain-manager](https://github.com/amplify-education/serverless-domain-manager) from 6.2.0 to 7.3.8.
- [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](https://github.com/amplify-education/serverless-domain-manager/commits/v7.3.8)

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

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