Update sharp optional dependency to support NPM 7

See lovell/sharp#2419 and prebuild/prebuild-install#128

Author: jorrit

Diff for: packages/next/package.json

@@ -112,7 +112,7 @@
     "react-dom": "^16.6.0 || ^17"
   },
   "optionalDependencies": {
-    "sharp": "0.26.2"
+    "sharp": "0.26.3"
   },
   "devDependencies": {
     "@babel/code-frame": "7.10.4",