fix: docfield of sales_order is not fetching route options for new doc (#21123)

Using the wrong method to get `so` as docfield.
frappe.meta changes will not effect the `sales_order` in `frm`.
diff --git a/erpnext/projects/doctype/project/project.js b/erpnext/projects/doctype/project/project.js
index 3570a0f..5862963 100644
--- a/erpnext/projects/doctype/project/project.js
+++ b/erpnext/projects/doctype/project/project.js
@@ -18,7 +18,7 @@
 		};
 	},
 	onload: function (frm) {
-		var so = frappe.meta.get_docfield("Project", "sales_order");
+		var so = frm.get_docfield("Project", "sales_order");
 		so.get_route_options_for_new_doc = function (field) {
 			if (frm.is_new()) return;
 			return {
@@ -135,4 +135,4 @@
 		frappe.ui.form.make_quick_entry(doctype, null, null, new_doc);
 	});
 
-}
\ No newline at end of file
+}