fix: improved warning message for legacy non-reactive properties

[benmccann]

This does make it a bit on the long side. However, previously the second sentence was just a restatement of the first and didn't tell you what to watch out for leading to confusion around the need for the message

closes #14111
closes #13811
closes #14034

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14190-svelte.vercel.app/

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Playground

pnpm add https://pkg.pr.new/svelte@14190

[benmccann]

Closing in favor of #14192