Update dependency eslint to ^7.15.0

Author: renovate-bot

package-lock.json

@@ -26,7 +26,7 @@
   ],
   "devDependencies": {
     "@wordpress/eslint-plugin": "^7.4.0",
-    "eslint": "^7.12.1",
+    "eslint": "^7.15.0",
     "grunt": "~1.3.0",
     "grunt-contrib-clean": "~2.0.0",
     "grunt-contrib-compress": "^1.6.0",