- Bump api-platform/serializer min to ^4.3.7 in JsonApi, Hal and GraphQl
  composer.json so they pull a serializer release that exposes the new
  protected isCacheKeySafe() method on AbstractItemNormalizer.
- Change createCacheKeySafeProbe() return type to object so PHPStan can
  resolve probeIsCacheKeySafe() on the anonymous AbstractItemNormalizer
  subclass.