Add geo_pipeline_replication feature flag

What does this MR do and why?

Adds a feature flag so pipeline ref replication can be disabled.

My hypothesis is that the logic is unneeded. Therefore we can disable it in staging-ref and Dedicated (which should immediately alleviate https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/incident-management/-/issues/128).

We should also confirm that it is not needed in combination with https://docs.gitlab.com/ee/administration/geo/secondary_proxy/runners.html.

If those tests show that it is not needed, then we can delete the code.

#438632 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

This MR does not cause a behavior change. You must disable the FF in order to cause a behavior change. The following steps are how we can validate the hypothesis. I have SSL set up locally and use Podman since I can't use Docker, and both things combined make it really hard to set up a Runner locally.

1. Set up Geo
2. Enable secondary runners
3. Set up a Runner with the secondary Geo site
4. Push changes to a repo
5. Confirm that the pipeline works
6. In rails console disable the feature flag

   Feature.disable(:geo_pipeline_replication)

7. Push changes to a repo
8. Confirm that the pipeline works