refactor: add xmlns to phpunit config

Author: Dominik Jansen

phpunit.xml.dist

@@ -1,5 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<phpunit bootstrap="vendor/autoload.php"
+<phpunit
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.4/phpunit.xsd"
+    bootstrap="vendor/autoload.php"
     backupGlobals="false"
     backupStaticAttributes="false"
     colors="true"