[Feature flag] Enable keeping latest artifact for ref

What

Enable the :keep_latest_artifacts_for_ref and :destroy_only_unlocked_expired_artifacts feature flags.

Owners

• Team: CI
• Most appropriate slack channel to reach out to: #g_ci
• Best individual to reach out to: @matteeyah

Expectations

What are we expecting to happen?

• All new pipelines should have locked set to artifacts_locked
• When a pipeline succeeds it should unlock previous pipelines for the same ref
• When destroying artifacts, only artifacts whose pipelines are unlocked should be destroyed

What might happen if this goes wrong?

• ExpireBuildArtifactsWorker might start failing due to database timeouts
• Ci::PipelineSuccessUnlockArtifactsWorker might start failing due to database timeouts
• Ci::RefDeleteUnlockArtifactsWorker might start failing due to database timeouts

What can we monitor to detect problems with this?

https://dashboards.gitlab.com/d/000000278/postgresql-slow-queries?orgId=1&refresh=1m

Beta groups/projects

• Small sample project
• gitlab-org/gitlab project
• gitlab-org/gitlab-com groups

Roll Out Steps

• Enable on staging
• Test on staging
• Ensure that documentation has been updated
• Coordinate a time to enable the flag with #production and #g_delivery on slack.
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production
• Cross post chatops slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel