fix(starters): update dependency prop-types to ^15.7.1 (#11686)

Author: renovate[bot]

starters/default/package-lock.json

@@ -13,7 +13,7 @@
     "gatsby-plugin-sharp": "^2.0.20",
     "gatsby-source-filesystem": "^2.0.20",
     "gatsby-transformer-sharp": "^2.1.13",
-    "prop-types": "^15.6.2",
+    "prop-types": "^15.7.1",
     "react": "^16.8.0",
     "react-dom": "^16.8.0",
     "react-helmet": "^5.2.0"