Improve the code responsible to perform import

This issue is to track effort of simplifying the import/export code, by reducing its complexity:

• Removing ci_pipelines.iid hack: !18003 (merged)
• Reduce amount of SQL queries related to SELECT * FROM project: !18005 (merged),
• Improve complexity of project update in order to remove hacks: !18007 (merged),
• Improve the readability and performance of project and relations restore: !18024 (merged),
• Rework Import/Export versioning mechanism: make it possible to import older compatible exports: #35861 (closed)
• Expand project Import / Export test coverage and ensure we use data set including all project features: #35341 (closed)
• Remove RelationFactory::OVERRIDES, use reflection instead: !19433 (closed)
• Strong validate import_export.yml to disallow exporting _id columns: #35339 (closed)
• Drop UNIQUE_RELATIONS edge case: #36647 (closed)
• De-dup data: !18026 (closed)
• etc.

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