Merge pull request #26327 from asmorkalov:as/drop_convertFp16

Finally dropped convertFp16 function in favor of cv::Mat::convertTo() #26327 

Partially address https://github.com/opencv/opencv/issues/24909
Related PR to contrib: https://github.com/opencv/opencv_contrib/pull/3812

### Pull Request Readiness Checklist

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

- [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
- [x] The PR is proposed to the proper branch
- [x] 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/core/test/test_arithm.cpp

@@ -924,8 +924,16 @@ struct ConvertScaleFp16Op : public BaseElemWiseOp
     void op(const vector<Mat>& src, Mat& dst, const Mat&)
     {
         Mat m;
-        convertFp16(src[0], m);
-        convertFp16(m, dst);
+        if (src[0].depth() == CV_32F)
+        {
+            src[0].convertTo(m, CV_16F);
+            m.convertTo(dst, CV_32F);
+        }
+        else
+        {
+            src[0].convertTo(m, CV_32F);
+            m.convertTo(dst, CV_16F);
+        }
     }
     void refop(const vector<Mat>& src, Mat& dst, const Mat&)
     {
@@ -935,7 +943,7 @@ struct ConvertScaleFp16Op : public BaseElemWiseOp
     {
         // 0: FP32 -> FP16 -> FP32
         // 1: FP16 -> FP32 -> FP16
-        int srctype = (nextRange & 1) == 0 ? CV_32F : CV_16S;
+        int srctype = (nextRange & 1) == 0 ? CV_32F : CV_16F;
         return srctype;
     }
     void getValueRange(int, double& minval, double& maxval)