chore: Halve the number of routes necessary to serve prerendered files on Vercel

[elliott-with-the-longest-name-on-github]

Previously, for a route like /blog/[slug] where /blog/first-post is prerendered with trailingSlash: 'always', we output the following routing:

{
	"version": 3,
	"routes": [
                 {
                         "src": "/blog/first-post/",
                         "dest": "/blog/first-post"
                 },
		{
			"src": "/blog/first-post",
			"status": 308,
			"headers": {
				"Location": "/about/"
			}
		},
		...
	],
	"overrides": {
		"blog/first-post/index.html": {
			"path": "blog/first-post"
		}
	}
}

The original code from #7725 assumes that the first routing entry is necessary to get /blog/first-post/ to match the path from the overrides, but it's not -- both blog/first-post and blog/first-post/ match this path, so all we really have to do is make sure we deploy a redirect route that sends the user to the correct variant. The new version of this routing code looks like this:

{
	"version": 3,
	"routes": [
		{
			"src": "/blog/first-post",
			"status": 308,
			"headers": {
				"Location": "/about/"
			}
		},
		...
	],
	"overrides": {
		"blog/first-post/index.html": {
			"path": "blog/first-post"
		}
	}
}

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🦋 Changeset detected

Latest commit: b8b1758

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-vercel	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13989-svelte.vercel.app/

[elliott-with-the-longest-name-on-github]

Vercel may actually be adding trailing slash normalization to static file overrides, so we may be able to sidestep this issue entirely. Closing this for now to avoid the unnecessary work.