Finalize mark_admin_bot_runners_as_hosted batched background migration

This batched background migration was added in !183329 (diffs)

The next required stop is 18.5 per https://docs.gitlab.com/update/upgrade_paths/ , so the migration can be finalized in 18.6 according to our migration style guide: https://docs.gitlab.com/development/database/batched_background_migrations/#finalize-a-batched-background-migration

This came up while changing all Users::Internal call-sites to use per-organization internal users. That MR should not affect the functionality or behavior of the BBM, and finalizing the BBM is not required for the changes to internal users going forward. However, out of an abundance of caution, we may hold off releasing the feature for self-managed until all affected BBMs are finalized.