Ok. I did some more checking, and it turns out that the namespace attribute is optional (it's not even used in SMF 1.1), so you can safely remove it...
Sources/News.php
Find
<rss version=', $xml_format == 'rss2' ? '"2.0" xmlns="http://backend.userland.com/rss2"' : '"0.92"', ' xml:lang="', strtr($txt['lang_locale'], '_', '-'), '">
Replace
<rss version=', $xml_format == 'rss2' ? '"2.0"' : '"0.92"', ' xml:lang="', strtr($txt['lang_locale'], '_', '-'), '">
That should make the RSS 2.0 feed valid as well.