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/stanza_error.rs

@@ -245,7 +245,7 @@ pub struct StanzaError {
 
     /// Human-readable description of this error.
     #[xml(extract(n = .., namespace = ns::XMPP_STANZAS, name = "text", fields(
-        attribute(name = "xml:lang", type_ = Lang, default),
+        lang(type_ = Lang, default),
         text(type_ = String),
     )))]
     pub texts: BTreeMap<Lang, String>,