docs: preserve async migration history

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: janechu

packages/fast-element/MIGRATION.md

@@ -213,6 +213,7 @@ This is a **breaking change** for SSR output format. Any system that produces or
     // Before
     MyElement.defineAsync({
         name: "my-element",
+        templateOptions: "defer-and-hydrate",
     });
 
     // After