feat!: drop legacy configs, requires ESLint v9 and ESM

[antfu]

This PR is intended to be released as v1.0.

In this PR:

• Drops he legacy .eslintrc format support
• It now requires ESLint v9.0 with the flat configs
• Drops the CJS build of the plugins and configs (now they are ESM only)
• Removes the legacy Nuxt 2 configs (which have not been released for a very long while)

This PR is more like a clean-up.

For users that use @nuxt/eslint module with the eslint.config.mjs config, everything should keep working as-is with no migration needed.

This change will break the usage for users that are using our new config with legacy eslintrc format, for example:

module.exports = {
  root: true,
  extends: ["@nuxt/eslint-config"],
};

...which I expect should be a rare case, and in the worst case, they can still use the existing 0.7.x.

[antfu]

/cc @danielroe bringing you the awareness in case if you have any opinions on this move, thanks!

[danielroe]

That change seems good to me 👌