Deprecate the functionPerRoute option (#11728)

* Deprecate the functionPerRoute option

* Update .changeset/weak-dancers-beam.md

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

---------

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

.changeset/weak-dancers-beam.md

@@ -0,0 +1,20 @@
+---
+'@astrojs/vercel': minor
+---
+
+Deprecates the `functionPerRoute` option
+
+This option is now deprecated, and will be removed entirely in Astro v5.0. We suggest removing this option from your configuration as soon as you are able to:
+
+```diff
+import { defineConfig } from 'astro/config';
+import vercel from '@astrojs/vercel/serverless';
+
+export default defineConfig({
+  // ...
+  output: 'server',
+  adapter: vercel({
+-     functionPerRoute: true,
+  }),
+});
+```

packages/integrations/vercel/src/serverless/adapter.ts

@@ -134,7 +134,10 @@ export interface VercelServerlessConfig {
 	/** Whether to create the Vercel Edge middleware from an Astro middleware in your code base. */
 	edgeMiddleware?: boolean;
 
-	/** Whether to split builds into a separate function for each route. */
+	/**
+	 * Whether to split builds into a separate function for each route.
+	 * @deprecated `functionPerRoute` is deprecated and will be removed in the next major release of the adapter.
+	 */
 	functionPerRoute?: boolean;
 
 	/** The maximum duration (in seconds) that Serverless Functions can run before timing out. See the [Vercel documentation](https://vercel.com/docs/functions/serverless-functions/runtimes#maxduration) for the default and maximum limit for your account plan. */
@@ -286,6 +289,11 @@ export default function vercelServerless({
 							`\tMake sure to check your plan carefully to avoid incurring additional costs.\n` +
 							`\tYou can set functionPerRoute: false to prevent surpassing the limit.\n`,
 					);
+
+					logger.warn(
+						`\n` +
+							`\t\`functionPerRoute\` is deprecated and will be removed in a future version of the adapter.\n`
+					);
 				}
 
 				setAdapter(