Add pipelineTriggersDelete GraphQL mutation

What does this MR do and why?

Part of #346054 (closed)

This MR adds a new mutation to DELETE a pipelineTrigger.

🚨 Note: This MR is dependent on and to be merged after: !126537 (merged) - because this MR is on the community fork, I can't set the upstream branch to md-update-pipeline-trigger, but we can view the compared branches HERE instead. Hopefully that makes reviewing and locating only the newly changed files a bit easier!

Related MRs:

• Get all pipeline triggers: !125887 (merged)
• Create pipeline trigger: !125887 (merged)
• Update pipeline trigger: !126537 (merged)

Screenshots or screen recordings

pipelines before running delete mutation

running the delete mutation

after running the delete mutation - note the trigger is gone!

How to set up and validate locally

1. Run gdk/gitpod server
2. Visit http://127.0.0.1:3000/-/graphql-explorer
3. Query all triggers for a project, copy the gid of any trigger you'd like

query {
  project(fullPath: "gitlab-org/gitlab-shell") {
    pipelineTriggers {
      edges {
        node {
          id
          description
        }
      }
    }
  }
}

4. Run a delete mutation on the trigger whose gid you copied

mutation {
  pipelineTriggerDelete(
    input: {id: "gid://gitlab/Ci::Trigger/10"}
  ) {
    errors
  }
}

5. Query all triggers again and verify the trigger was deleted

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.