Fix compatibility on optional key on array dim fetch to be Mixed on phpstan patch 2.1.x-dev

[samsonasik]

@VincentLanglet this should fixes rectorphp/rector#9770

1) Rector\Tests\TypeDeclaration\Rector\ClassMethod\AddMethodCallBasedStrictParamTypeRector\AddMethodCallBasedStrictParamTypeRectorTest::test#20 with data ('/home/runner/work/phpstan/php...hp.inc')
Failed on fixture file "skip_native_optional_array_shape.php.inc"
Failed asserting that string matches format description.
--- Expected
+++ Actual
@@ @@
         $this->doBar($shape['dirname']); // dirname is only conditionally returned
     }
 
-    private function doBar($param) {
+    private function doBar(string $param) {
 
     }
 }

The other notice is only union order so I think we can just update the fixture once next phpstan released.

-    public static function extension($filename): array|string
+    public static function extension($filename): string|array

step to reproduce:

1. Temporary change composer.json phpstan/phpstan require to 2.1.x-dev
2. Run unit test

[samsonasik]

All green 🎉 @TomasVotruba let's merge ;)