breaking: remove support for Cloudflare Pages

[teemingc]

closes #15060

We will eventually need to do this when Cloudflare officially sunsets Cloudflare Pages or when we add integration for the Cloudflare Vite plugin anyway. Doing it earlier will make the latter easier.

This PR:

• removes the test app for cloudflare pages
• changes the auto adapter to detect the Cloudflare Workers Builds environment instead
• adds a check to the Cloudflare adapter to tell users to migrate to Cloudflare Workers

---

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 2138166:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/2138166848da32e6cbba90dabe6daf898dba3de6

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16604

[changeset-bot]

🦋 Changeset detected

Latest commit: 2138166

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/adapter-auto	Major
@sveltejs/adapter-cloudflare	Major

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16604-svelte.vercel.app/

[Nic-Polumeyv]

Suggested change

	* Only for Cloudflare Pages. Customize the automatically-generated [`_routes.json`](https://developers.cloudflare.com/pages/platform/functions/routing/#create-a-_routesjson-file) file.
	* @deprecated
	* @deprecated Cloudflare Pages is no longer supported

the fallback JSDoc above also still has the Pages paragraphs

[Nic-Polumeyv]

Suggested change

	'@sveltejs/adapter-cloudflare no longer supports Cloudflare Pages. Please follow the migration guide to move your project to Cloudflare Workers https://developers.cloudflare.com/workers/static-assets/migration-guides/migrate-from-pages/'
	'@sveltejs/adapter-cloudflare no longer supports Cloudflare Pages. Please follow the migration guide to move your project to Cloudflare Workers https://developers.cloudflare.com/workers/static-assets/migration-guides/migrate-from-pages/ If you have already migrated, delete the leftover `_routes.json` file from your project root.'

for people who migrated but kept a stale _routes.json

[Nic-Polumeyv]

parse_redirects lost its only caller, can go too (plus its tests)