Improve n+1 in pipeline serializer for triggered pipelines

What does this MR do?

Improves the query count for pipeline_serializer by 10

• Adds a latest_statuses association
• Adds the association to the preloads in pipeline_serializer

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Part of #225156 (closed)