docs: Explicitly mention flat config for findability

[Sidnioulz]

No linked issue.

What Changed

This PR adds an explicit mention of "flat config" in the flat config section.

Why?

I often search for 'flat config' when taking over a project and having to decide if it's ready to upgrade to ESLint 9. When I looked at this plugin today, I parsed the README quickly and initially missed the flat config section. I had to check the dependencies to see that it supports ESLint 9.

Many major ESLint plugins use the word flat somewhere in their description of this new config:

• https://typescript-eslint.io/getting-started/
• https://www.npmjs.com/package/eslint-plugin-prettier
• https://www.npmjs.com/package/eslint-plugin-import
• https://www.npmjs.com/package/eslint-plugin-turbo
• https://eslint.vuejs.org/user-guide/

The ones that don't:

• https://nextjs.org/docs/app/api-reference/config/eslint doesn't yet support it but gives FlatCompat examples
• https://www.npmjs.com/package/eslint-config-airbnb doesn't yet support it

Using the explicit wording in the relevant section ensures a ^+F search finds a match, since not finding anything when searching for "flat" could lead users to wrongly assume that ESLint 9 is not supported.

Checklist

Check the ones applicable to your change:

• Ran pnpm run update-all
• Tests are updated
• Documentation is updated

Change Type

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• maintenance
• documentation
• patch
• minor
• major

[github-actions]

🚀 PR was released in v0.11.3 🚀