Fix type annotation for shopware/platform:v6.4.15.0

HttpKernel.php

diff --git a/src/HttpKernel.php b/src/HttpKernel.php
index bafa884..db29ad0 100644
--- a/src/HttpKernel.php
+++ b/src/HttpKernel.php
@@ -4,5 +4,5 @@ namespace Shopware\Production;

 class HttpKernel extends \Shopware\Core\HttpKernel
 {
-    protected static $kernelClass = Kernel::class;
+    protected static string $kernelClass = Kernel::class;
 }