Fix "'elementAt' is deprecated and shouldn't be used. Use operator + instead."

Author: greenrobot

objectbox/lib/src/native/bindings/helpers.dart

@@ -183,7 +183,7 @@ T withNativeStrings<T>(
     return fn(ptr, size);
   } finally {
     for (var i = 0; i < size; i++) {
-      malloc.free(ptr.elementAt(i).value);
+      malloc.free((ptr + i).value);
     }
     malloc.free(ptr);
   }

objectbox/lib/src/native/query/query.dart

@@ -1149,7 +1149,7 @@ class Query<T> implements Finalizable {
       final count = resultPtr.ref.count;
       return List.generate(count, (i) {
         // items[i] only available with Dart 3.3
-        final item = items.elementAt(i).ref;
+        final item = (items + i).ref;
         final id = item.id;
         final score = item.score;
         return IdWithScore(id, score);