Update dependency @types/jest to v26

Author: renovate-bot

package.json

@@ -39,7 +39,7 @@
     "@rollup/plugin-json": "^4.0.2",
     "@types/amphtml-validator": "^1.0.1",
     "@types/css-tree": "^1.0.3",
-    "@types/jest": "^25.1.4",
+    "@types/jest": "26.0.23",
     "gts": "^1.1.2",
     "jest": "^25.2.0",
     "jest-environment-jsdom-fourteen": "^1.0.1",