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/disco.rs

@@ -57,7 +57,7 @@ pub struct Identity {
     pub type_: String,
 
     /// Lang of the name of this identity.
-    #[xml(attribute(default, name = "xml:lang"))]
+    #[xml(lang(default))]
     pub lang: Option<String>,
 
     /// Name of this identity.