Fix passing of `dry_run` to pipeline simulation

What does this MR do and why?

Bug fix for #337282 (closed). See #219416 (comment 1052836723) for context.

The pipeline simulation feature should pass the dry_run: true to the CI Lint endpoint. This MR fixes the typo where the endpoint accepts the dry parameter instead of dry_run.

Screenshots or screen recordings

How to set up and validate locally

1. Go to the pipeline editor. (CI/CD > Editor)
2. In the Edit tab, paste the following content:

   pre-checks:
     stage: .pre
     script:
       - echo "running a job from .pre stage"
   
   post-checks:
     stage: .post
     script:
       - echo "running a job from .post stage"

3. In the Validate tab, click on Validate simulation
4. Instead of a success message, we should receive an error message that says No stages / jobs for this pipeline.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.