Cleanup `run_pipeline_graphql` feature flag

What does this MR do and why?

This enables the run_pipeline_graphql feature flag by default and updates the CI schema linting to cover the variables:options keyword.

The feature flag is already enabled for all projects in production.

Rollout: #372310 (closed)

Screenshots or screen recordings

Update CI Schema Linting

Pre-filled dropdown in the Run Pipeline form

How to set up and validate locally

1. Make sure the feature flag is disabled (rails c > Feature.disable(:run_pipeline_graphql))

2. Add the following to your .gitlab-ci.yml config file:

   stages:
     - test
   
   variables:
     DEPLOY_ENVIRONMENT:
       value: "development"
       options:
         - "production"
         - "development"
         - "staging" 
       description: "The deployment target. Change this variable to 'canary' or 'production' if needed."
   
   declare-config-vars:
     stage: test
     script:
       - echo $DEPLOY_ENVIRONMENT

3. Go to CI/CD > Pipelines then click on the Run Pipeline button on the upper right corner.

4. Verify that the variable DEPLOY_ENVIRONMENT is pre-filled in the form with the value "development". The dropdown list will have the same values as in options.

5. Click on the Run Pipeline button. This will create the pipeline with your variables and redirect you to the newly created pipeline.

6. Wait for the pipeline to finish then check the job logs. The correct variable values should be printed in the logs.

MR acceptance checklist

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