[Feature flag] Enable Pipeline Triggers settings port to Vue

What

In #250312 (closed) we delivered a change to rewrite the Pipeline Triggers table in Vue (MR !41864 (merged))

The goal of this rollout issue is to ensure the Vue port works as the previous version and then remove the previous version.

Remove the :ci_pipeline_triggers_settings_vue_ui feature flag.

Cleanup notes:

Owners

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

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
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour
• Coordinate a time to enable the flag with #production and #g_delivery on slack.
• Clean up old code (non-exhaustive list)
  • app/presenters/ci/trigger_presenter.rb (full removal)
  • spec/features/triggers_spec.rb (remove old tests)
  • Remove @triggers from app/controllers/projects/settings/ci_cd_controller.rb
• 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