COMP: Fix -Wdeprecated-declarations in ctkDICOMThumbnailGenerator

This commit fixes the following warning:

/path/to/CTK/Libs/DICOM/Widgets/ctkDICOMThumbnailGenerator.cpp:174:23: warning: ‘QByteArray& QByteArray::append(const QString&)’ is deprecated: Use QString's toUtf8(), toLatin1() or toLocal8Bit() [-Wdeprecated-declarations]
  174 |   buffer.append(header);
      |                       ^

Co-authored-by: James Butler <james.butler@perkinelmer.com>
Co-authored-by: Andras Lasso <lasso@queensu.ca>

Libs/DICOM/Widgets/ctkDICOMThumbnailGenerator.cpp

@@ -171,7 +171,7 @@ bool ctkDICOMThumbnailGenerator::generateThumbnail(DicomImage *dcmImage, QImage&
   /* create output buffer for DicomImage class */
   QByteArray buffer;
   /* copy header to output buffer and resize it for pixel data */
-  buffer.append(header);
+  buffer.append(header.toUtf8());
   buffer.resize(length);
 
   /* render pixel data to buffer */