production: Increase loader-metadata parallelism from 4 to 6

https://grafana.softwareheritage.org/goto/AENkFouVk?orgId=1 shows most of the time is spent fetching from origins (so storage isn't a bottleneck), and https://grafana.softwareheritage.org/goto/fJViKou4z?orgId=1 shows we still have plenty of quota left.