Deprecated code removed and updated with new.

Closes #3236444

src/Normalizer/ContentEntityNormalizer.php

@@ -247,7 +247,7 @@ class ContentEntityNormalizer extends NormalizerBase implements DenormalizerInte
     $translations = [];
     foreach ($data as $key => $translation) {
       // Skip any keys that start with '_' or '@'.
-      if (in_array($key{0}, ['_', '@'])) {
+      if (in_array($key[0], ['_', '@'])) {
         continue;
       }
       // When language is configured, undefined or not applicable go ahead with