Update RSS schema

src/invidious.cr

@@ -1939,7 +1939,8 @@ get "/feed/channel/:ucid" do |env|
 
   feed = XML.build(indent: "  ", encoding: "UTF-8") do |xml|
     xml.element("feed", "xmlns:yt": "http://www.youtube.com/xml/schemas/2015",
-      "xmlns:media": "http://search.yahoo.com/mrss/", xmlns: "http://www.w3.org/2005/Atom") do
+      "xmlns:media": "http://search.yahoo.com/mrss/", xmlns: "http://www.w3.org/2005/Atom",
+      "xml:lang": "en-US") do
       xml.element("link", rel: "self", href: "#{host_url}#{path}")
       xml.element("id") { xml.text "yt:channel:#{ucid}" }
       xml.element("yt:channelId") { xml.text ucid }
@@ -2060,7 +2061,8 @@ get "/feed/private" do |env|
   query = env.request.query.not_nil!
 
   feed = XML.build(indent: "  ", encoding: "UTF-8") do |xml|
-    xml.element("feed", "xmlns:yt": "http://www.w3.org/2005/Atom", "xmlns:media": "http://search.yahoo.com/mrss/",
+    xml.element("feed", "xmlns:yt": "http://www.youtube.com/xml/schemas/2015",
+      "xmlns:media": "http://search.yahoo.com/mrss/", xmlns: "http://www.w3.org/2005/Atom",
       "xml:lang": "en-US") do
       xml.element("link", "type": "text/html", rel: "alternate", href: "#{host_url}/feed/subscriptions")
       xml.element("link", "type": "application/atom+xml", rel: "self", href: "#{host_url}#{path}?#{query}")