PUT MergeRequest API endpoint - accept labels as an array

What does this MR do?

Accepts labels sent as an array ['bug', 'feature freeze'] as well as the usual comma-separated string ("bug, feature freeze") in the PUT MergeRequests API endpoint.

Despite this addition I believe documentation should not be added declaring we support this. Instead this should be a "principle of least surprise" addition (since we also have some other endpoints in which we expect comma-separated strings and don't support arrays and we should update those as well if this gets added)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

It seems like a point of confusion that we have to send what a developer would usually think of as an array in a comma-separated list

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?