www/gitlab: security and bugfix update to 18.3.1
Changes: https://about.gitlab.com/releases/2025/08/27/patch-release-gitlab-18-3-1-released/
Security: d7b7e505-8486-11f0-9d03-2cf05da270f3
(cherry picked from commit de841c6124da3f3662c82667ffbcfc91a74aea33)