[Feature flag] Rollout of `new_pipeline_form_prefilled_vars`

What

Remove the :new_pipeline_form_prefilled_vars feature flag, which preloads values from the gitlab-ci.yml so users can immediately click on "Run pipeline", see #30101 (closed) for more details.

!43392 (merged), !44120 (merged)

a fix: !47189 (merged)

Owners

• Team: ~"group::continuous integration"
• Most appropriate slack channel to reach out to: #g_ci
• Best individual to reach out to: @mrincon , @furkanayhan

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

• Enable on staging (/chatops run feature set feature_name 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 feature_name true)
• 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 (/chatops run feature set feature_name 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