Fix cross-DB operations on dast_site_profile_id

MR introducing the exception: !163154 (merged)

Related issue: #472353 (closed)

Files affected:

• spec/lib/gitlab/database/no_cross_db_foreign_keys_spec.rb (cross-DB foreign key)
• ee/spec/lib/gitlab/database/desired_sharding_key_spec.rb (cross-DB join)
• spec/support/shared_examples/lib/gitlab/background_migration/backfill_desired_sharding_key_shared_examples.rb (cross-DB join)
• ee/app/services/app_sec/dast/site_profiles/create_service.rb (cross DB transaction)
• ee/app/services/app_sec/dast/site_profiles/update_service.rb (cross DB transaction)

The two tables depending on dast_site_profiles are: dast_site_profile_secret_variables and dast_site_profiles_builds