Update frontmatter with ai-usage and current date

Co-authored-by: gewarren <[email protected]>

Author: Copilot

docs/standard/serialization/system-text-json/nullable-annotations.md

@@ -1,8 +1,9 @@
 ---
 title: Respect nullable annotations
 description: "Learn how to configure serialization and deserialization to respect nullable annotations."
-ms.date: 10/22/2024
+ms.date: 10/20/2025
 no-loc: [System.Text.Json, Newtonsoft.Json]
+ai-usage: ai-assisted
 ---
 # Respect nullable annotations