Delete project subscription mutation

What does this MR do and why?

This MR enables Project/Pipeline Subscription Deletion through the GraphQL API

Database Review

The destroy operation yields the following query:

    DELETE FROM "ci_subscriptions_projects"
    WHERE "ci_subscriptions_projects"."id" = 19

Query Plan: https://console.postgres.ai/gitlab/gitlab-production-ci/sessions/23313/commands/74931

Screenshots or screen recordings

Success

Non-existing Subscription

How to set up and validate locally

• Checkout the branch and start the application
• Make sure that you are using your EE license
• Open a public project and navigate to CI/CD settings on the left panel
• Create a few project subscriptions using other public projects
• Open the GraphQL explorer
• Ping the mutation

An sample graphql query would be the following

mutation {
  deleteProjectSubscription(input: {
    subscriptionId: "gid://gitlab/Ci::Subscriptions::Project/7"
  } ) {
    project {
      name
    }
    errors
  }
}

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.

Related to #425286 (closed)