Update dependency eslint to v7

package.json

@@ -22,7 +22,7 @@
   "devDependencies": {
     "enzyme": "3.11.0",
     "enzyme-adapter-preact-pure": "2.2.0",
-    "eslint": "6.8.0",
+    "eslint": "7.8.1",
     "eslint-config-synacor": "3.0.5",
     "identity-obj-proxy": "3.0.0",
     "jest": "25.1.0",