Provide packages for Debian 13 (Trixie)

Debian 13 is planned to be released on August 9th. Once that is out, GitLab should start providing packages following the policy https://handbook.gitlab.com/handbook/engineering/infrastructure-platforms/gitlab-delivery/distribution/workflow/#os

Definition of Done

• Ensure Packagecloud supports the Distribution
• Create a builder image for Debian 13 in https://gitlab.com/gitlab-org/gitlab-omnibus-builder. Previous example - gitlab-omnibus-builder!288 (merged)
• Add CI jobs to run unit tests and build packages. Previous example - !6968 (merged)
• Add Debian 13 to the supported OS list - https://docs.gitlab.com/install/package/#supported-platforms
  • Add to the list in https://docs.gitlab.com/install/package/debian/ also.