fix: update dependency async to ^2.6.3 (#16806)

Author: renovate[bot]

packages/gatsby-plugin-sharp/package.json

@@ -8,7 +8,7 @@
   },
   "dependencies": {
     "@babel/runtime": "^7.0.0",
-    "async": "^2.1.2",
+    "async": "^2.6.3",
     "bluebird": "^3.5.0",
     "fs-extra": "^7.0.0",
     "gatsby-core-utils": "^1.0.4",