Resolve "Disabling Pipelines for a project does not disable "Only allow merge requests to be merged if the pipeline succeeds""

What does this MR do?

FE: Always show the "only merge when pipeline succeeds" checkbox in a project's merge request settings, even if the pipelines feature for the project is disabled.

BE: Ignore the "only merge when pipeline succeeds" setting if the project's pipelines feature is disabled. But do not change it.

EE compat: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5996

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

Why was this MR needed?

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #39206 (moved)