Merge branch '6.3' into 6.4

* 6.3:
  [Config] Prefixing FileExistenceResource::__toString() to avoid conflict with FileResource
  [String] Method toByteString conversion using iconv is unreachable
  Accept mixed key on DsPairStub
  [HttpKernel] Fix PHP deprecation

Caster/DsPairStub.php

@@ -18,7 +18,7 @@
  */
 class DsPairStub extends Stub
 {
-    public function __construct(string|int $key, mixed $value)
+    public function __construct(mixed $key, mixed $value)
     {
         $this->value = [
             Caster::PREFIX_VIRTUAL.'key' => $key,