Add previousStageJobs to JobType

What does this MR do and why?

This MR adds previousStageJobs field to Ci::JobType. This field will be used with the existing needs field to provide a more performant replace to the previousStageJobsOrNeeds field.

This MR also fixes an existing N+1 in the previous_stage_jobs method. This removes the N+1 from both the new previousStageJobs field and from previousStageJobsOrNeeds

Issues: #424255 (closed), #389195 (closed), #422079 (closed)

Screenshots or screen recordings

The above query comes from a pipeline run using this .gitlab-ci.yml:

build:
  script: ls
  stage: build

test:
  script: ls
  stage: test

deploy:
  script: ls
  stage: deploy

How to set up and validate locally

1. Run a pipeline using this .gitlab-ci.yml:

   build:
     script: ls
     stage: build
   
   test:
     script: ls
     stage: test
   
   deploy:
     script: ls
     stage: deploy

2. Make this query in /-/graphql-explorer

   {
     project(fullPath: PROJECT_FULL_PATH) {
       pipeline(iid: PIPELINE_IID) {
         jobs {
           nodes {
             name
             previousStageJobs {
               nodes {
   	    name
               }
             }
           }
         }
       }
     }
   }

3. See that you get the result:

{
  "data": {
    "project": {
      "pipeline": {
        "jobs": {
          "nodes": [
            {
              "name": "deploy",
              "previousStageJobs": {
                "nodes": [
                  {
                    "name": "test"
                  }
                ]
              }
            },
            {
              "name": "test",
              "previousStageJobs": {
                "nodes": [
                  {
                    "name": "build"
                  }
                ]
              }
            },
            {
              "name": "build",
              "previousStageJobs": {
                "nodes": []
              }
            }
          ]
        }
      }
    }
  }
}

MR acceptance checklist

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