Use partial index on `merge_request_id`

What does this MR do?

We added merge_request_id column to ci_pipelines table, however, more than half of the rows will not have merge_request_id. We should switch to partial index in order to reduce the Disk IO operation by writing indexes.

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/54913

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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