chore: throw helpful migration error (#8915)

For the Vercel and Netlify adapters. adapter-auto on earlier versions could install them when it shouldn't
Related to #8911

.changeset/large-tables-breathe.md

@@ -0,0 +1,6 @@
+---
+'@sveltejs/adapter-netlify': patch
+'@sveltejs/adapter-vercel': patch
+---
+
+chore: throw helpful migration error

packages/adapter-netlify/index.js

@@ -39,6 +39,13 @@ export default function ({ split = false, edge = edge_set_in_env_var } = {}) {
 		name: '@sveltejs/adapter-netlify',
 
 		async adapt(builder) {
+			if (!builder.routes) {
+				throw new Error(
+					'@sveltejs/adapter-netlify >=2.x (possibly installed through @sveltejs/adapter-auto) requires @sveltejs/kit version 1.5 or higher. ' +
+						'Either downgrade the adapter or upgrade @sveltejs/kit'
+				);
+			}
+
 			const netlify_config = get_netlify_config();
 
 			// "build" is the default publish directory when Netlify detects SvelteKit

packages/adapter-vercel/index.js

@@ -26,6 +26,13 @@ const plugin = function (defaults = {}) {
 		name: '@sveltejs/adapter-vercel',
 
 		async adapt(builder) {
+			if (!builder.routes) {
+				throw new Error(
+					'@sveltejs/adapter-vercel >=2.x (possibly installed through @sveltejs/adapter-auto) requires @sveltejs/kit version 1.5 or higher. ' +
+						'Either downgrade the adapter or upgrade @sveltejs/kit'
+				);
+			}
+
 			const dir = '.vercel/output';
 			const tmp = builder.getBuildDirectory('vercel-tmp');