Swagger: Fix response for the search users endpoint

[papagian]

What is this feature?

[Add a brief description of what the feature or update does.]

Why do we need this feature?

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Who is this feature for?

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Which issue(s) does this PR fix?:

Fixes #71106

Special notes for your reviewer:
to review the changes:
start grafana server and navigate to:
http://localhost:3000/swagger-ui#/users/searchUsers
http://localhost:3000/swagger-ui#/users/searchUsersWithPaging

in the specification there are additional changes because in #65241
the enterprise endpoints were removed (the specification was generated without enterprise enabled) and re-introduced in this one

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.