Truncate p_ci_build_name partitions that are not 103

As a follow-up for !188496 (merged)

Create a batched background migration to run only on Gitlab.com

This migration will truncate partitions that are not 103 - the partitions 102 and older are at least 1 month old. This'll limit searching builds names to only the last month for now... And eventually the last 3 months once the worker is enabled.

After the migration MR is merged; we can enable the FF: truncate_build_names