Add final modifier

Co-authored-by: Lam Tran <lam.tran@commercetools.com>

Author: salander85

src/main/java/com/commercetools/sync/products/ProductSync.java

@@ -258,7 +258,7 @@ private CompletionStage<Void> syncOrKeepTrack(
                   final Set<String> missingReferencedProductKeys =
                       getMissingReferencedProductKeys(newDraft, keyToIdCache);
 
-                  boolean selfReferenceExists =
+                  final boolean selfReferenceExists =
                       missingReferencedProductKeys.remove(newDraft.getKey());
 
                   if (!missingReferencedProductKeys.isEmpty()) {