Update dependency eslint to v7.21.0

Author: renovate-bot

webapp/package-lock.json

@@ -27,7 +27,7 @@
     "@polymer/test-fixture": "4.0.2",
     "babel-eslint": "10.1.0",
     "cpy-cli": "3.1.1",
-    "eslint": "7.20.0",
+    "eslint": "7.21.0",
     "eslint-plugin-html": "6.1.1",
     "wct-browser-legacy": "1.0.2",
     "wct-local": "2.1.5",