COMP: Update legacy boolean ivar set method call to current API

Update legacy boolean ivar set method call to current API: the
`itk::VectorGradientMagnitudeImageFilter::m_UseImageSpacing` ivar got the
boolean macro methods in commit c6610df,
and the former methods were marked as legacy code.

Fixes:
```
/Users/builder/externalExamples/IO/VisibleHumanPasteWrite.cxx:72:9:
error: no member named 'SetUseImageSpacingOn' in
'itk::VectorGradientMagnitudeImageFilter<itk::Image<itk::RGBPixel<unsigned char>, 2>, float, itk::Image<float, 2> >'
  grad->SetUseImageSpacingOn();
  ~~~~  ^
1 error generated.
```

Raised for example at:
https://open.cdash.org/viewBuildError.php?buildid=6964751

Author: jhlegarreta

Examples/IO/VisibleHumanPasteWrite.cxx

@@ -69,7 +69,7 @@ main(int argc, char * argv[])
     GradientMagnitudeImageFilter::New();
   grad->SetInput(reader->GetOutput());
 
-  grad->SetUseImageSpacingOn();
+  grad->UseImageSpacingOn();
 
   using GradientMagnitudeOutputImageType =
     GradientMagnitudeImageFilter::OutputImageType;