xmpp-parsers: Switch to #[xml(lang)] wherever it’s possible

So far this new meta doesn’t work for the BTreeMap<Lang, String>
pattern, which is often used to provide multiple versions of a payload.

skip-changelog: This isn’t a user-facing change.

parsers/src/websocket.rs

@@ -32,7 +32,7 @@ pub struct Open {
 
     /// The default human language for all subsequent stanzas, which will
     /// be transmitted to other entities for better localisation.
-    #[xml(attribute(default, name = "xml:lang"))]
+    #[xml(lang(default))]
     pub xml_lang: Option<String>,
 }