[Feature flag] Rollout of `retarget_merge_requests`

What

Remove the :retarget_merge_requests feature flag introduced with !53710 (merged).

Owners

• Team: Memory
• Most appropriate slack channel to reach out to: #g_memory
• Best individual to reach out to: @ayufan

Expectations

What are we expecting to happen?

MRs that have remove source branch will trigger to retarget automatically MRs targeting the branch to be removed.

What might happen if this goes wrong?

The retarget does not happen.

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

• gitlab-org/gitlab project: /chatops run feature set --project=gitlab-org/gitlab retarget_merge_requests true
• gitlab-org/gitlab-runner project: /chatops run feature set --project=gitlab-org/gitlab-runner retarget_merge_requests true
• gitlab-org/charts/gitlab-runner project: /chatops run feature set --project=gitlab-org/charts/gitlab-runner retarget_merge_requests true
• gitlab-org/ci-cd/custom-executor-drivers/autoscaler project: /chatops run feature set --project=gitlab-org/ci-cd/custom-executor-drivers/autoscaler retarget_merge_requests true
• gitlab-org/ci-cd/shared-runners/images/macstadium/orka project: /chatops run feature set --project=gitlab-org/shared-runners/images/macstadium/orka retarget_merge_requests true
• gitlab-org/gitlab-pages project: /chatops run feature set --project=gitlab-org/gitlab-pages retarget_merge_requests true

Roll Out Steps

• Enable on staging (/chatops run feature set retarget_merge_requests true --staging)
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour (/chatops run feature set --project=gitlab-org/gitlab retarget_merge_requests true)
• Coordinate a time to enable the flag with the SRE oncall and release managers
  • In #production mention @sre-oncall and @release-managers. Once an SRE on call and Release Manager on call confirm, you can proceed with the rollout
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production (/chatops run feature set retarget_merge_requests true)
• 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

Rollback Steps

• This feature can be disabled by running the following Chatops command:

/chatops run feature set --project=gitlab-org/gitlab retarget_merge_requests false