[Feature flag] Rollout for the `alert_runbooks` feature.

What

Remove the :alert_runbooks feature flag ...

MR to add the feature flag: !37910 (merged)

Owners

• Team: devopsmonitor -> ~"group::health"
• Most appropriate slack channel to reach out to: #g_monitor_health
• Best individual to reach out to: @tristan.read

Expectations

### What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?

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
• gitlab-org/gitlab-com groups
• ...

Roll Out Steps

• [n/a] Enable on staging
• [n/a] Test on staging
• Ensure that documentation has been updated MR here !38661 (merged)
• [n/a] Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• [n/a] Coordinate a time to enable the flag with #production and #g_delivery on slack.
• [n/a] Announce on the issue an estimated time this will be enabled on GitLab.com
• [n/a] Enable on GitLab.com by running chatops command in #production
• [n/a] Cross post chatops slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• [n/a] Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry MR here !39315 (merged)
• [n/a] After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel