chore: upgrade eslint to v9

[jrmajor]

Blocked by / test for sveltejs/eslint-config#31.

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[benmccann]

why did this change?

[jrmajor]

ESLint reported always true condition. 'non-empty-string' || (whatever) will always return 'non-empty-string', unless I'm missing something obvious.

[benmccann]

Ah, got it. I guess that's fine then. This has probably been like that for quite awhile, so should be safe to cleanup