Fix cross-join between dast_site_validations and dast_site_tokens

Related issue: #472350 (closed)

Related MR that introduced the cross-joins/cross-database transactions: !160199 (merged)

Files affected:

• ee/app/models/dast_site.rb
• ee/app/models/dast_site_validation.rb
• ee/app/services/app_sec/dast/site_validations/revoke_service.rb
• lib/gitlab/background_migration/backfill_desired_sharding_key_job.rb