COMP: Use ImageRegionIteratorWithIndex for SSIM output (legacy-removed)

The output iterator in StructuralSimilarityImageFilter was a plain
ImageRegionIterator and the per-pixel loop called outIt.GetIndex()
to test interior-region membership for the mean accumulator.  The
GetIndex() method on the index-less iterator is wrapped in
ITK_FUTURE_LEGACY_REMOVE and emits a deprecation warning that
CI's Linux legacy-removed build promotes to an error.

Switch the output iterator to ImageRegionIteratorWithIndex so the
index is tracked natively, eliminating the deprecation entirely.

Author: hjmjohnson

Modules/Filtering/ImageCompare/include/itkStructuralSimilarityImageFilter.hxx

@@ -22,7 +22,7 @@
 #include "itkDiscreteGaussianImageFilter.h"
 #include "itkImage.h"
 #include "itkImageRegionConstIterator.h"
-#include "itkImageRegionIterator.h"
+#include "itkImageRegionIteratorWithIndex.h"
 #include "itkMath.h"
 #include "itkMultiplyImageFilter.h"
 #include "itkProgressReporter.h"
@@ -259,7 +259,11 @@ StructuralSimilarityImageFilter<TInputImage, TOutputImage>::GenerateData()
   this->GetMultiThreader()->template ParallelizeImageRegion<ImageDimension>(
     outputRegion,
     [&](const OutputImageRegionType & subRegion) {
-      using OutputIteratorType = ImageRegionIterator<OutputImageType>;
+      // Use ImageRegionIteratorWithIndex for the output so we can call
+      // GetIndex() inside the inner loop without hitting the
+      // ITK_LEGACY_REMOVE deprecation on the index-less iterator's
+      // GetIndex() (#6034 CI fix).
+      using OutputIteratorType = ImageRegionIteratorWithIndex<OutputImageType>;
       using RealConstIteratorType = ImageRegionConstIterator<RealImageType>;
 
       OutputIteratorType    outIt(output, subRegion);