Draft: Increase container registry alert threshold

We recently enabled the feature flag container_registry_migration_phase2_capacity_40 which increases the number of repositories that can be migrated at a time. This recently caused the alert ContainerRegistryPhase2RailsStalledPreImporting to go off. However, this was expected because we are currently targeting repositories with higher number of layers, meaning they can potentially take longer to migrate.

This MR increases the average to 15 instead of 10, which should give us some room to allow repositories with more layers to complete in time; without triggering the alert.

Related to gitlab-org/gitlab#350920 (closed)