vercel: use Node.js 20 runtime in adapter to fix invalid runtime deployment error

Author: gitcoder89431

svelte.config.js

@@ -6,7 +6,7 @@ const config = {
 	preprocess: vitePreprocess(),
 	kit: {
 		adapter: adapter({
-			runtime: 'nodejs18.x',
+			runtime: 'nodejs20.x',
 		}),
 	},
 };