chore(deps): migrate from eslint-plugin-markdown to @eslint/markdown (#2500)

## PR Checklist

- [x] Addresses an existing open issue: fixes #2499
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/TypeStat/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/TypeStat/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

The other package has been deprecated for some time. Take the
recommended package into use.

🐙

Author: rubiesonthesky

eslint.config.js

@@ -10,10 +10,10 @@ If you're interested in learning more, see the 'getting started' docs on:
 
 import comments from "@eslint-community/eslint-plugin-eslint-comments/configs";
 import eslint from "@eslint/js";
+import markdown from "@eslint/markdown";
 import vitest from "@vitest/eslint-plugin";
 import jsdoc from "eslint-plugin-jsdoc";
 import jsonc from "eslint-plugin-jsonc";
-import markdown from "eslint-plugin-markdown";
 import n from "eslint-plugin-n";
 import packageJson from "eslint-plugin-package-json/configs/recommended";
 import perfectionist from "eslint-plugin-perfectionist";
@@ -40,7 +40,6 @@ export default defineConfig(
 	},
 	eslint.configs.recommended,
 	...jsonc.configs["flat/recommended-with-json"],
-	...markdown.configs.recommended,
 	...yml.configs["flat/recommended"],
 	...yml.configs["flat/prettier"],
 	comments.recommended,
@@ -106,12 +105,20 @@ export default defineConfig(
 			},
 		},
 	},
+	{
+		extends: ["markdown/processor"],
+		files: ["**/*.md"],
+		plugins: {
+			markdown,
+		},
+	},
 	{
 		extends: [tseslint.configs.disableTypeChecked],
 		files: ["**/*.md/*.*s"],
 		rules: {
 			"@typescript-eslint/no-unused-vars": "off",
 			"n/no-missing-import": "off",
+			"no-undef": "off",
 		},
 	},
 	{

package.json

@@ -51,8 +51,8 @@
 	"devDependencies": {
 		"@eslint-community/eslint-plugin-eslint-comments": "4.7.1",
 		"@eslint/js": "10.0.1",
+		"@eslint/markdown": "8.0.1",
 		"@release-it/conventional-changelog": "11.0.0",
-		"@types/eslint-plugin-markdown": "2.0.2",
 		"@types/node": "22.19.17",
 		"@types/prop-types": "15.7.15",
 		"@types/react": "19.2.14",
@@ -63,7 +63,6 @@
 		"eslint": "10.2.1",
 		"eslint-plugin-jsdoc": "62.9.0",
 		"eslint-plugin-jsonc": "3.1.2",
-		"eslint-plugin-markdown": "5.1.0",
 		"eslint-plugin-n": "17.16.1",
 		"eslint-plugin-package-json": "0.26.0",
 		"eslint-plugin-perfectionist": "5.9.0",