[12.x] Use full path to Str in exception markdown (#56705)

Otherwise showing an exception will fail with an exception, if the
facade could not be found.

This error might have been introduced in v12.25.0

Author: apreiml

src/Illuminate/Foundation/resources/exceptions/renderer/markdown.blade.php

@@ -13,7 +13,7 @@
 
 ## Request
 
-{{ $exception->request()->method() }} {{ Str::start($exception->request()->path(), '/') }}
+{{ $exception->request()->method() }} {{ \Illuminate\Support\Str::start($exception->request()->path(), '/') }}
 
 ## Headers