Cleanup `graphql_job_app` feature flag

What does this MR do and why?

This enables the graphql_job_app feature flag by default and removes legacy code in the job app. The feature flag is already enabled for all projects in production.

This is a follow up of !96199 (merged) and will update the state of the feature flag to be permanent.

Notes

• It's a big diff! Most of it is just removed code.
• The feature should function exactly the same as the production version.
• The most important file to look at is manual_variables_form.vue. These updates combined the logic of legacy_manual_variables_form. with this file to consolidate the API logic.
• Second file that contributes to additions is job_app_spec.js because I needed to update it due to API stuff. I'll finish that refactor in a follow up MR.

Screenshots or screen recordings

No screenshots as the feature functions the exact same.

How to set up and validate locally

• Navigate to CI/CD > Editor
• Set up a manual job in your pipeline editor. Assuming your have 2 variables, varOne and varTwo, you can use this:

manual_job:
  script:
    - echo "$varOne" "$varTwo"
  when: manual
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

• Navigate to the manual job after running a pipeline.
• You should see an empty state with the manual variable form and a Trigger button (empty state before screenshot). Enter some variables and trigger the job.
• After the job runs, instead of a blue rerun button, you should see a dropdown. From here, go through the flow of the designs and make sure the feature behaves as expected.
• Compare this behavior to the same flow in staging or production

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.