Merge pull request #28058 from nklskyoy:onnx-importer2-dispatch-map

Onnx importer2 dispatch map #28058

I have noticed that PR[28032]( https://github.com/opencv/opencv/pull/28032) was incomplete - it fixed only the new ONNX importer. Also, building the dispatch map based on the parsed opset version hypotetically can cause trouble if the graph simplifier inserts a node with a different opset version which is not included in the dispatch map. So I removed this parameter in `buildDispatchMap_COM_MICROSOFT` and `buildDispatchMap_ONNX_AI` for now. It was marked as `CV_UNUSED` anyway. 

### Pull Request Readiness Checklist

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- [x] I agree to contribute to the project under Apache 2 License.
- [x] 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

modules/dnn/src/net_impl.cpp

@@ -64,7 +64,7 @@ Net::Impl::Impl()
 
     modelFormat = DNN_MODEL_GENERIC;
     originalLayout = DATA_LAYOUT_NCHW;
-    onnx_opset = 0;
+    // onnx_opset = 0;
 
     accuracy = CV_32F;
     enableFP16 = haveFP16 = false;