feat: drop ESLint v7 support

BREAKING CHANGE: drops ESLint v7 support

Author: gajus

package.json

@@ -51,7 +51,7 @@
   "main": "./dist/index.js",
   "name": "eslint-plugin-flowtype",
   "peerDependencies": {
-    "eslint": "^7.0.0 || ^8.0.0"
+    "eslint": "^8.0.0"
   },
   "repository": {
     "type": "git",