Refactor fetchDiffFilesMeta & fetchDiffFiles

What does this MR do?

Related to: #202663

Refactor fetchDiffFiles and fetchDiffFilesMeta to not return any data.

Screenshots

n/a

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content