fix: update dependency core-js to ^2.6.9 (#16839)

Author: renovate[bot]

packages/gatsby-cli/package.json

@@ -21,7 +21,7 @@
     "common-tags": "^1.4.0",
     "configstore": "^4.0.0",
     "convert-hrtime": "^2.0.0",
-    "core-js": "^2.5.0",
+    "core-js": "^2.6.9",
     "envinfo": "^5.8.1",
     "execa": "^0.8.0",
     "fs-exists-cached": "^1.0.0",

packages/gatsby/package.json

@@ -43,7 +43,7 @@
     "compression": "^1.7.4",
     "convert-hrtime": "^2.0.0",
     "copyfiles": "^1.2.0",
-    "core-js": "^2.5.0",
+    "core-js": "^2.6.9",
     "cors": "^2.8.5",
     "css-loader": "^1.0.0",
     "debug": "^3.1.0",