LibWeb/XML: Set attributes using Element::set_attribute_ns

Otherwise we'd end up ignoring attr prefixes and treating them as part
of the attribute's local name.
Fixes #3137.

This is a partial revert of b3fdeef,
but it fixes the underlying issue that it was working around.

Libraries/LibWeb/XML/XMLDocumentBuilder.cpp

@@ -142,12 +142,13 @@ void XMLDocumentBuilder::element_start(const XML::Name& name, HashMap<XML::Name,
             } else {
                 m_has_error = true;
             }
-        } else if (attribute.key.contains(":"sv)) {
-            if (!attribute.key.starts_with("xml:"sv)) {
-                m_has_error = true;
-            }
         }
-        MUST(node->set_attribute(MUST(FlyString::from_deprecated_fly_string(attribute.key)), MUST(String::from_byte_string(attribute.value))));
+
+        auto result = node->set_attribute_ns(m_namespace, MUST(FlyString::from_deprecated_fly_string(attribute.key)), MUST(String::from_byte_string(attribute.value)));
+        if (result.is_error()) {
+            m_has_error = true;
+            return;
+        }
     }
 
     m_current_node = node.ptr();