Description:
This pull request adds the CV_WRAP annotation to the cv::dnn::Net::registerOutput method to make it accessible from language bindings such as Python and Java.

Background
The methods getUnconnectedOutLayers and getUnconnectedOutLayersNames are deprecated, and registerOutput is the recommended alternative. However, it is currently not available from wrappers, making migration difficult.

Changes
Added CV_WRAP to Net::registerOutput in dnn.hpp.

Notes
This change does not affect existing functionality and only exposes the method to binding generators.

Please let me know if further adjustments are needed.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake