Resolve CE to EE differences in the lib/api directory

What does this MR do?

This MR contains changes backported from or necessary by EE (in the CE code) to resolve all differences in the lib/api directory.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9633

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9505

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer