Use Graphql mutation for CI Lint for validate pipeline and pipeline simulation pages

What does this MR do and why?

This MR updates the home page of the pipeline editor to use the GraphQl mutation for CI lint, instead of the deprecated GraphQL query. There shouldn't be any visual differences.

References

• #540765 (closed)
• !197153 (merged)

Screenshots or screen recordings

Before	After
Screen_Recording_2025-07-09_at_15.22.27	Screen_Recording_2025-07-09_at_15.25.49
Screen_Recording_2025-07-09_at_15.23.38	Screen_Recording_2025-07-09_at_15.25.14

How to set up and validate locally

1. Create .gitlab-ci.yml file with config in your project
2. Visit Build -> Pipeline editor
3. Select Validate tab
4. Verify the GitLab CI configuration is displayed correctly.
5. From the same page, select Lint CI/CD sample button.
6. Verify the sample is displayed correctly.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #540765 (closed)