Merge pull request #26590 from Kumataro:fix26589

Support C++20 standard #26590

Close https://github.com/opencv/opencv/issues/26589
Related https://github.com/opencv/opencv_contrib/pull/3842
Related: https://github.com/opencv/opencv/issues/20269

- do not arithmetic enums and ( different enums or floating numeric) 
- remove unused variable

### 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
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/videoio/src/cap_gstreamer.cpp

@@ -961,7 +961,7 @@ bool GStreamerCapture::retrieveVideoFrame(int, OutputArray dst)
     // regular strides/offsets if no meta is present.
     GstVideoFrame frame = {};
 #if FULL_GST_VERSION >= VERSION_NUM(1,6,0)
-    GstMapFlags flags = static_cast<GstMapFlags>(GST_MAP_READ | GST_VIDEO_FRAME_MAP_FLAG_NO_REF);
+    GstMapFlags flags = static_cast<GstMapFlags>(+GST_MAP_READ | GST_VIDEO_FRAME_MAP_FLAG_NO_REF);
 #else
     GstMapFlags flags = static_cast<GstMapFlags>(GST_MAP_READ);
 #endif