Draft: Remove ee/spec/models/security/finding_spec.rb from cross-join allowlist

What does this MR do and why?

This fixes a simple cross-join in has_one :build as well as allowlisting directly the callsite of a known cross-join issue. This allows us to remove a whole test file from an allowlist as it no longer contains offences.

I only found 1 usage (outside of the test) which was https://gitlab.com/gitlab-org/gitlab/-/blob/532e3e3e9a0042c7a5a27add11c4257b5fad8806/ee/app/finders/security/findings_finder.rb#L77 . I didn't check if there were preloads but the only reliable way to do that is to run the whole pipeline anyway so a green pipeline should indicate this was never being used for preloading as it won't work with preloads anymore.

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Closes #342380 (closed)