Fix auto_cancel_pending_pipelines check

What does this MR do?

It fixes auto_cancel_pending_pipelines check

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

In Rails 5 auto_cancel_pending_pipelines? is not a valid method for checking whether auto_cancel_pending_pipelines is enabled or disabled. Auto-generated helper methods are only for enum values, so in this case it would be project.enabled? or project.disabled?. The problem is that these helper method are too generic and such check would be completely misleading, so rather than use project.enabled?, an explicit check == 'enabled' is used.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47834 (closed)