Fix broken background migration spec

What does this MR do and why?

ee/spec/lib/ee/gitlab/background_migration/populate_uuids_for_security_findings_spec.rb is broken because !84117 (merged) introduced a delegate :project, to: :pipeline and the migration had it's own implementation of the Ci::Pipeline model which didn't have the Project association defined

How to set up and validate locally

bundle exec rspec ee/spec/lib/ee/gitlab/background_migration/populate_uuids_for_security_findings_spec.rb

MR acceptance checklist

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