xmpp-parsers: Use the new lang meta everywhere

Stop parsing xml:lang manually, instead use the #[xml(lang)] meta which
tracks it using the XML semantics, inheriting it from the most recent
ancestor.

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

parsers/src/sasl.rs

@@ -157,7 +157,7 @@ pub struct Failure {
 
     /// A human-readable explanation for the failure.
     #[xml(extract(n = .., name = "text", fields(
-        attribute(type_ = String, name = "xml:lang", default),
+        lang(type_ = Lang, default),
         text(type_ = String),
     )))]
     pub texts: BTreeMap<Lang, String>,