Add pipelineTriggersUpdate GraphQL mutation

What does this MR do and why?

Part of #346054 (closed)

This MR adds a new mutation to UPDATE a pipelineTrigger.

Related MRs:

• Get all pipeline triggers: !125887 (merged)
• Create pipeline trigger: !125887 (merged)
• Delete pipeline trigger: !126540 (merged)

Screenshots or screen recordings

pipelines before running update mutation - note the description

running the update mutation

after running the update mutation - can see the pipeline description is changed

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 an update mutation on the trigger whose gid you copied

mutation {
  pipelineTriggerUpdate(
    input: {id: "gid://gitlab/Ci::Trigger/10", description: "A new engaging description, huzzah!"}
  ) {
    pipelineTrigger {
      id
      description
    }
    errors
  }
}

5. Query all triggers again and verify the trigger description was updated

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.