Add OpenAPI documentation for Ci::SecureFiles

This MR adds API::Ci::SecureFiles to OpenAPI V2.

• Check the API::Ci::SecureFiles class:
  • Check if the parameter documentation needs to be updated
  • Check if the description block needs to be updated. Ideally this should include a success entry.
    • [-] If the endpoint responds with an Array, use the is_array true helper.
  • Compare the parameters and description to the markdown documentation for this endpoint
• Add the endpoint to the OpenApiV2 documentation by modifying lib/api/api.rb and moving mount Lint into the other namespace, next to add_open_api_documentation!
• Run rake gitlab:openapi:generate
• Review changes in doc/api/openapi/openapi_v2.yaml. You can check the content of the file by pasting it into https://editor.swagger.io. Errors will be shown on the left side.
• Please do not commit the changes in doc/api/openapi/openapi_v2.yaml. Many teams will work on this in parallel. To avoid conflicts, we will try to have only one MR modifying the file at a time. @elim-ext or

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/376880#note_1162776112

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.