chore: replace -prefetch with -preload-code in tests

[wackbyte]

Replaces the old data-sveltekit-prefetch attribute with data-sveltekit-preload-code in certain tests. This removes all mention of the attribute from the repository. (other than changelogs, of course)

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[benmccann]

looks like this change happened prior to 1.0, so I don't think we were keeping the old version around to test against it or anything like that. we could probably use any attribute here, but makes sense to update it anyway
https://github.com/sveltejs/kit/blob/main/packages/kit/CHANGELOG.md#100-next565