Provide packages for OpenSUSE Leap 15.6

Summary

OpenSuse 15.6 was released on June 12, 2024, but we do not provide GitLab packages for it: the latest versions available at https://packages.gitlab.com/app/gitlab/gitlab-ee/ are for 15.5.

Proposal

Please provide GitLab packages compatible with OpenSuse 15.6.

Steps

1. ✅ Ensure Packagecloud supports it: #8752 (comment 2145792815)
2. ✅ Create builder images at https://gitlab.com/gitlab-org/gitlab-omnibus-builder: gitlab-omnibus-builder!359 (merged)
3. ✅ Add CI jobs to build and release the packages using the builder images: !7993 (merged)
4. ✅ Update supported OSes documentation: gitlab!169600 (merged)