`eslint-plugin-patternfly-react` - Add support for flat config format for ESLint

Currently the eslint-plugin-patternfly-react supports the ESLint v9 plugin API thanks to the work done by @kmcfaul under #10743. However it still requires the use of compatibility functions in order to transform the legacy configuration format to something that can be used in ESLint v9 due to its use of the now required flat configuration format.

The implementation of eslint-plugin-patternfly-react should be updated so it supports the new configuration format, and the documentation in the README should be updated to reflect that. For an example of this, see the documentation for the ESLint markdown plugin.

eslint-plugin-patternfly-react の実装とその README を見つけ、設定ガイダンスをリンク先の eslint-plugin-markdown の例と比較します。ESLint の flat configuration format をサポートするようにプラグインを更新し、新しい使用方法を README に記載します。実装とドキュメントの両方が互換性変換なしで ESLint v9 で動作すれば完了です。