Add API::UserCounts endpoints to OpenApiV2 documentation

What does this MR do and why?

This adds the API::UserCounts endpoints to the OpenAPI V2 documentation.

How to set up and validate locally

• Run bin/rake gitlab:openapi:generate to see the change in the swagger yaml file.
• See #377006 for more details.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377006