Rename visit_preorder to visit_source_order (#17046)

## Summary

We renamed the `PreorderVisitor` to `SourceOrderVisitor` a long time ago
but it seems that we missed to rename the `visit_preorder` functions to
`visit_source_order`.
This PR renames `visit_preorder` to `visit_source_order`

## Test Plan

`cargo test`

crates/ruff_python_ast/src/generated.rs

@@ -6027,7 +6027,7 @@ impl AnyNodeRef<'_> {
 }
 
 impl<'a> AnyNodeRef<'a> {
-    pub fn visit_preorder<'b, V>(self, visitor: &mut V)
+    pub fn visit_source_order<'b, V>(self, visitor: &mut V)
     where
         V: crate::visitor::source_order::SourceOrderVisitor<'b> + ?Sized,
         'a: 'b,