Remove restriction on xmlns attributes

Author: tvdijen

src/Attribute.php

@@ -32,7 +32,6 @@ public function __construct(
         protected string $attrValue,
     ) {
         Assert::nullOrStringNotEmpty($namespaceURI);
-        Assert::notSame('xmlns', $namespacePrefix);
         if ($namespaceURI !== null) {
             Assert::stringNotEmpty($namespacePrefix);
         }