[Sec Infra] Backfill migration for Reachability field

After syncs linked as blocking issues are addressed. Perform Backfill migration for the Vulnerabilities ES index for the existing SBOM records with reachability.

Along with this also add to the GraphQL resolvers whether the migration has completed check, this check be in ee/app/graphql/resolvers/vulnerability_filterable.rb which is being implemented in !194804 (merged)