Backfill has_issues for vulnerability_reads rows that have an external issue tracker

Why are we doing this work

As outlined in &12882 (comment 1782997506) we need to perform a batched background migration to update rows in vulnerability_reads table if there's an existing record in vulnerability_external_issue_links

Implementation plan

1. Create and implement a batched background migration using guidelines - https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#how-to