Mark HasSource::source_old as deprecated but allow at all call sites

crates/completion/src/render/function.rs

@@ -34,6 +34,7 @@ impl<'a> FunctionRender<'a> {
         fn_: hir::Function,
     ) -> FunctionRender<'a> {
         let name = local_name.unwrap_or_else(|| fn_.name(ctx.db()).to_string());
+        #[allow(deprecated)]
         let ast_node = fn_.source_old(ctx.db()).value;
 
         FunctionRender { ctx, name, func: fn_, ast_node }