fix: note old eslint plugin deprecation

[dummdidumm]

closes #10242

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[changeset-bot]

🦋 Changeset detected

Latest commit: 00b6e0f

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

This PR includes changesets to release 1 package

Name	Type
svelte-migrate	Patch

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[benmccann]

should we make a note in the migration guide as well?

[geoffrich]

It's already there: https://svelte.dev/docs/v4-migration-guide#new-eslint-package