minor #48079 Fix the DataTransformerInterface generic types to support the patcher (stof)

This PR was merged into the 6.2 branch.

Discussion
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Fix the DataTransformerInterface generic types to support the patcher

| Q             | A
| ------------- | ---
| Branch?       | 6.2
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       |
| License       | MIT
| Doc PR        | n/a

The type patcher adding return types handles template types only for methods that already have a return type. As `DataTransformerInterface` is not typed yet in Symfony 6.x, we need to keep the future native types in ``@return`` for the patcher tool. This puts the generic type (introduced in #47412) in ``@psalm`-return`, as done in #48012 (I detected this issue when regenerating the return type for that other PR)

Commits
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d9fd5c71ad Fix the DataTransformerInterface generic types to support the patcher

Author: nicolas-grekas

DataTransformerInterface.php

@@ -58,7 +58,9 @@ interface DataTransformerInterface
      *
      * @param TValue|null $value The value in the original representation
      *
-     * @return TTransformedValue|null
+     * @return mixed
+     *
+     * @psalm-return TTransformedValue|null
      *
      * @throws TransformationFailedException when the transformation fails
      */
@@ -87,7 +89,9 @@ public function transform(mixed $value);
      *
      * @param TTransformedValue|null $value The value in the transformed representation
      *
-     * @return TValue|null
+     * @return mixed
+     *
+     * @psalm-return TValue|null
      *
      * @throws TransformationFailedException when the transformation fails
      */