When a pipeline for a merge request was exported, the merge request was
not properly mapped, which caused a validation error, "Validation
failed: Merge request can't be blank".

Since neither merge requests nor CI pipelines have an explicit
relationship in `import_export.yml`, the full merge_request hash was
never exported. To make these older exports work, we now process
`merge_request_id` by replacing it with the newly-imported ID.

Closes https://gitlab.com/gitlab-org/gitlab/issues/31414