fix: fixtures/blocks/package.json & fixtures/blocks/yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-BABELRUNTIMECOREJS3-9397696

Author: snyk-bot

fixtures/blocks/package.json

@@ -8,7 +8,7 @@
     "json-server": "^0.16.1",
     "react": "^16.13.1",
     "react-dom": "^16.13.1",
-    "react-scripts": "3.4.1"
+    "react-scripts": "4.0.0"
   },
   "scripts": {
     "prestart": "cp -r ../../build/node_modules/* ./node_modules/",