Backfill the new runner taggings table

#472974 (closed) handles writing new data to ci_runner_taggings, but we still have a few billion rows in the taggings table that need to be copied over too.

We need to see if we can backfill any missing referenced ci_runners_e59bb2812d records as part of the migration, which would allow us to get started on this issue in %17.7 or %17.8. This would allow us to finalize the migration on %17.9. Otherwise, we'll need to split this issue into gitlab.com and self-managed issues, given that we have no guarantees that !166520 (merged) has completed on self-managed.

Proposal

Write a batched background migration to copy data from taggings into ci_runner_taggings. This migration should enrich the data from the taggings table with the sharding key.