Support ESLint 9.x

Similar to https://github.com/eslint/eslint-plugin-markdown/issues/194

It would be great to officially support ESLint v9.x now that it has been [released](https://eslint.org/blog/2024/04/eslint-v9.0.0-released/).

`eslint-plugin-markdown` is currently pinned to support `^6.0.0`, `^7.0.0`, or `^8.0.0`.