remove deprecations notices about DataTransformerInterface

Author: emmanuel-averty

src/Form/DataTransformer/StringToFileTransformer.php

@@ -29,7 +29,7 @@ public function __construct(?string $uploadDir, callable $uploadFilename, callab
  $this->filesystemOperator = $filesystemOperator;
  }
 
- public function transform($value)
+ public function transform($value): null|File|array
  {
  if (null === $value || [] === $value) {
  return null;
@@ -46,7 +46,7 @@ public function transform($value)
  return array_map([$this, 'doTransform'], $value);
  }
 
- public function reverseTransform($value)
+ public function reverseTransform($value): null|string|array
  {
  if (null === $value || [] === $value) {
  return null;