Fix project imports for pipelines for merge requests

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 #31414 (closed)