feat!: drop support for ESLint 4-8

BREAKING CHANGE: Minimum supported ESLint version is now 9.0.0.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: amilajack

README.md

@@ -21,27 +21,13 @@ npm install eslint-plugin-compat
 
 ### 2. Update ESLint Config
 
-#### New Config (`eslint.config.mjs`)
-
 ```js
+// eslint.config.mjs
 import compat from "eslint-plugin-compat";
 
 export default [compat.configs["flat/recommended"]];
 ```
 
-#### Legacy Config (`.eslintrc.json`)
-
-```jsonc
-{
-  "plugins": ["compat"],
-  "extends": ["plugin:compat/recommended"],
-  "env": {
-    "browser": true,
-  },
-  // ...
-}
-```
-
 ### 3. Configure Target Browsers
 
 Browser targets are configured using [browserslist](https://github.com/browserslist/browserslist). You can configure browser targets in your `package.json`:

package-lock.json

@@ -113,7 +113,7 @@
     "typescript-eslint": "^8.56.0"
   },
   "peerDependencies": {
-    "eslint": "^4.0.0 || ^5.0.0 || ^6.0.0 || ^7.0.0 || ^8.0.0 || ^9.0.0 || ^10.0.0"
+    "eslint": "^9.0.0 || ^10.0.0"
   },
   "engines": {
     "node": ">=18.x"