[Rules] Update settings.mdx

Per https://community.cloudflare.com/t/amp-and-signed-exchanges-deprecation-october-20th/831238

Author: nenizera

src/content/docs/rules/configuration-rules/settings.mdx

@@ -314,3 +314,7 @@ API configuration property name: `"sxg"` (boolean).
 <Render file="configuration-rule-link-to-examples" product="rules" />
 
 </Details>
+
+:::caution[Deprecation notice]
+Signed exchanges is being deprecated and will be removed on October 20, 2025.
+:::