Update dependency gts to v3

Author: renovate-bot

package.json

@@ -40,7 +40,7 @@
     "@types/amphtml-validator": "^1.0.1",
     "@types/css-tree": "^1.0.3",
     "@types/jest": "^25.1.4",
-    "gts": "^1.1.2",
+    "gts": "3.1.0",
     "jest": "^25.2.0",
     "jest-environment-jsdom-fourteen": "^1.0.1",
     "prettier": "^2.0.2",