[3.15] GitLab generate-supported-versions job #20267

Tracked under parent: #17809

Tier: B
Section: Tooling / CI

Not a cp315 wheel-ship blocker. GitLab job plumbing only.

Summary:

Land the GitLab generate-supported-versions job from #20100. The PR:

• sets SUPPORTED_VERSIONS_GENERATOR to scripts/integration_registry/generate_supported_versions.py in .gitlab-ci.yml
• points .gitlab/one-pipeline.locked.yml at the generate-supported-versions one-pipeline include (currently the WIP include-wip/dubloom/supported-version-job remote)
• adds a temporary generate-supported-versions rules override (when: manual, allow_failure: true) to validate that job on the branch

Done means that GitLab job plumbing is merged (the generator script is wired and the job exists in GitLab CI). The temporary manual / WIP-template pin is validation scaffolding on #20100, not extra product scope.

This is not the 3.15 supported_versions.json listing. That listing is #17814 / #19995 (Vlad).

To close this row, include Closes #<this-issue-number> on #20100 (or a follow-up that lands the same plumbing). That auto-closes this issue on merge, which auto-flips the corresponding checkbox on the parent tracker (#17809).