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/gapi/src/streaming/gstreamer/gstreamer_media_adapter.cpp

@@ -23,7 +23,7 @@ GStreamerMediaAdapter::GStreamerMediaAdapter(const cv::GFrameDesc& frameDesc,
 {
 #if GST_VERSION_MINOR >= 10
     // Check that GstBuffer has mono-view, so we can retrieve only one video-meta
-    GAPI_Assert((gst_buffer_get_flags(m_buffer) & GST_VIDEO_BUFFER_FLAG_MULTIPLE_VIEW) == 0);
+    GAPI_Assert((gst_buffer_get_flags(m_buffer) & static_cast<GstBufferFlags>(GST_VIDEO_BUFFER_FLAG_MULTIPLE_VIEW)) == 0);
 #endif // GST_VERSION_MINOR >= 10
 
     GstVideoMeta* videoMeta = gst_buffer_get_video_meta(m_buffer);