[Feature flag] Enable group deployment frequency API

What

Remove the :dora4_group_deployment_frequency_api feature flag ...

Owners

• Team: ~"group::release"
• Most appropriate slack channel to reach out to: #s_release
• Best individual to reach out to: @mkozono

Expectations

What are we expecting to happen?

When the flag is enabled, the group scope deployment frequency API will become available: !51938 (merged)

What might happen if this goes wrong?

It's possible that a DB query performed by the endpoint can be slow, see !51938 (comment 491179854) for more details.

What can we monitor to detect problems with this?

I'm not sure. @mayra-cabrera do you know this?

We could monitor the API for projects/:id/deployments (since it's using the same finder). Then once the monitor for Deployments Group is live, we should monitor that one instead.

Roll Out Steps

• Enable on staging (/chatops run feature set dora4_group_deployment_frequency_api true --staging)
• Test on staging
• Ensure that documentation has been updated
• 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 dora4_group_deployment_frequency_api 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 dora4_group_deployment_frequency_api false