Reenqeue NewMergeRequestWorker if not prepared

To ensure the NewMergeRequestWorker was run, we need to requeue this worker if the merge request is not yet prepared_at.

This may require some investigation the best place for this enqueuing to be executed from.

Availability and Testing

Regression testing, please ensure associated MR is labelled with pipeline:run-all-e2e and e2e:package-and-test job is passing.