Rename the attribute to on_unknown

compiler/rustc_span/src/symbol.rs

@@ -799,7 +799,7 @@
         diagnostic_namespace,
         diagnostic_on_const,
         diagnostic_on_move,
-        diagnostic_on_unknown_item,
+        diagnostic_on_unknown,
         dialect,
         direct,
         discriminant_kind,
@@ -1419,7 +1419,7 @@
         on_const,
         on_move,
         on_unimplemented,
-        on_unknown_item,
+        on_unknown,
         opaque,
         opaque_module_name_placeholder: "<opaque>",
         ops,