Re-enable platform-check

This makes sure that we output sensible error (not very pretty, but
useful) when required extensions are missing.

Fixes #7560

composer.json

@@ -68,7 +68,7 @@
         },
         "optimize-autoloader": true,
         "sort-packages": true,
-        "platform-check": false
+        "platform-check": true
     },
     "extra": {
         "branch-alias": {