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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