repo-updater: Use unnest instead of json_to_recordset

Created by: ryanslade

We found previously that using json_to_recordset can lead to temporary file limit errors.

In this case, even though we're mostly sending small amounts of data to our statements we have a trigger that runs after any rows in external_service_repos are deleted. The trigger reads most rows in our repo and external_service_repos table and this in combination with json_to_recordset appears to lead to temporary file limit errors.

Closes: https://github.com/sourcegraph/sourcegraph/issues/16318