docs: Mention package.json config support dropped

[nzakas]

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[x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
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What changes did you make? (Give an overview)

Added a note in the configuration migration guide that package.json configs are no longer supported.

fixes #18296

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Is there any place else that would make sense to mention this?

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[snitin315]

LGTM

[mahnunchik]

I won't upgrade to version 9 without support "eslintConfig" key in package.json 😢