Add migration guide for the deprecated analytics property

.changeset/sixty-teachers-tap.md

@@ -3,5 +3,24 @@
 ---
 
 Enable Vercel Speed Insights and Vercel Web Analytics individually.
+Deprecates the `analytics` property in `astro.config.mjs` in favor of `speedInsights` and `webAnalytics`.
+
+If you're using the `analytics` property, you'll need to update your config to use the new properties:
+
+```diff
+// astro.config.mjs
+export default defineConfig({
+	adapter: vercel({
+-   analytics: true,
++		webAnalytics: {
++			enabled: true
++		},
++		speedInsights: {
++			enabled: true
++		}
+	})
+});
+```
+
 Allow configuration of Web Analytics with all available configuration options.
 Bumps @vercel/analytics package to the latest version.