Resolve vulnerability cross transaction issues in `ee/app/services/security/ingestion/ingest_slice_base_service.rb`

What does this MR do and why?

This MR moves the marking of projects as containing vulnerabilities to a separate ingestion task transaction at the end of the vulnerability ingestion process to avoid cross database transactions which will not work after the sec-decomposition.

Nothing else about the functionality is affected.

MR acceptance checklist

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Related to #474635 (closed)