Add locked state to merge request API

What does this MR do?

Adds the locked state to the merge request API so that it can be used as a search filter.

Why was this MR needed?

For the case where an individual would want to search for locked merge requests.

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 Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#43790 (closed) - Add locked state to merge request API