TAP5-2789: fix error when compiling with Java 17

tapestry-version-migrator/src/main/java/org/apache/tapestry5/versionmigrator/Main.java

@@ -503,7 +503,7 @@ private static List<ClassRefactor> parse(List<String> lines)
     private static final class MavenNamespaceContext implements NamespaceContext {
 
         @Override
-        public Iterator<?> getPrefixes(String namespaceURI) 
+        public Iterator<String> getPrefixes(String namespaceURI) 
         {
             throw new UnsupportedOperationException();
         }