Rename ambiguous association names in Ci::Pipeline

What does this MR do?

Ci::Pipeline#merge_requests method returns merge requests which are associated with head pipelines. However, it's hard to guess from the association name how it behaves. In addition, we're going towards to "Pipelines for merge requests" architecture, which means each pipeline belongs to one merge request. In order to avoid the confusion across the transition period, we should give the old association name something explicit.

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

EE pipeline check

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9844

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer