Fix OpenAPI schema for merge request list endpoints

What does this MR do and why?

Fix OpenAPI schema for merge request list endpoints

Add is_array true flag to merge request list endpoint descriptions to correctly generate array schemas in OpenAPI specification.

This fixes the response schema for three endpoints that return arrays of merge requests but were incorrectly documented as returning single objects.

Changelog: fixed

References

Resolves: #506161 (closed)

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